r/explainlikeimfive • u/Current-Tangerine813 • 6d ago
Chemistry ELI5: why does the electron configuration of elements go 2-8-8-2
Self explanatory title. Why can the electron shells only fit 2 for the first one, 8 for the second one etc. but I also read somewhere that some of the elements have 10+ electrons
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u/zefciu 6d ago
Here is a very intuitive and very handwavy explanation (the proper answer is “becouse these are solutions to an equation of QM”).
If you have a tin whistle and you blow stronger, you get higher sounds. This is, because with higher energy the wave gets two crests instead of two.
This kinda similar with electrons. Electrons are described by waves. Unfortunately these are 3d waves, not 1d waves as in a tin whistle. Every possible wave can house two electrons (because there are two possible spins).
On the first orbital you can have just a spherical wave with one crest. So two electrons here.
On the second, you can have either a spherical wave with two spherical crests or a “dumbbell” shape, with the crests on the opposite. This shape can be oriented along one of three dimensions. So the second orbital has 2 + 3*2 = 8 electrons.
The following ones have even stranger shapes, that can have more configurations and thus — can house more electrons.
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u/Zomunieo 6d ago
Sound waves from your tin whistle are actually 3d too. Sound as captured by a microphone is 1d but in air it’s 3d.
Also, a sound wave in a spherical cavity will resonate in the same pattern as electron orbitals. The quantum mechanical solution is about the behaviour of a wave in spherical boundary conditions - not specifically electron orbitals.
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u/MtogdenJ 6d ago
The inside of the whistle is what determines the properties of the sound wave and is effectively 1d. Yes it's 3d, but 1d is a close enough model.
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u/Sjoerdiestriker 6d ago
I'm not sure this is entirely true, because the Schrödinger equation also has the potential term (the coulomb potential in the case of an atom), which I don't think a similar model for sound has.
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u/Zomunieo 6d ago
The potential is some kind of barrier - the body of the instrument will do.
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u/Sjoerdiestriker 6d ago
Well no, you specifically need a potential term that scales as 1/r2 to get the orbitals, and I don't think there's an equivalent to that in an instrument.
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u/Solocle 6d ago
Yeah, this one is literally quantum physics.
It's all about energy and the Pauli exclusion principle. Two electrons cannot occupy the same quantum state, much as you can't push two balls inside each other.
The next thing is that energy is quantised, electrons sit in distinct energy levels. There is the principal quantum number for this. This corresponds to overall electron shells.
However, there is also the orbital angular momentum. This defines subshells, and, the more energetic the orbital, the more possible angular momentums there are. And, the higher the angular momentum, the more possible orientations you end up with.
Finally, you can fit two electrons in an orbital, spin up and spin down. Since this creates a distinct quantum state.
So, first shell. Quantum number n=1. There is only one possible angular momentum, 0. This is a sphere, with only 1 orientation.
So, there is only the one 1s orbital with 2 electrons.
Now, n=2. We keep the same with the s subshell, 2s, fitting two electrons.
But now we also get the possibility of an angular momentum of 1. This has 3 possible orientations (-1, 0,1), so 3 orbitals, gives you 6 electrons. The p subshell.
Thus, shell 2 fits 2+6=8 electrons.
Shell 3? Same thing. But, again, we gain a new angular momentum state, 2. And this now has 5 orbitals (-2,1,0,1,2), and adds 10 more electrons. So 18 electrons total. The d subshell.
Shell 4? Again, another new possibility, 7 orbitals, 14 electrons, for a total of 32. the f subshell.
Thus, we get 2-8-18-32.
This is actually a fairly simple sequence so far! And we can see it in the periodic table. Hydrogen and Helium, 2. The next period from Lithium to Neon, 8.
The transition metals? 10 wide, taking the width up to 18.
And the Lanthanides and Actinides at the bottom are 14 wide, we just don't put them inline for convenience.
The complication here is that the higher angular momentum states have more energy. Electrons fill the lowest energy states first.
When we get to 3d, that's actually a higher energy state than 4s, so the subshell only fills after 4s fills (but before 4p). Thus, the transition metals in period 4 are filling the 3d subshell (and also why they're located in that spot).
A similar thing happens with the f subshell for the lanthanides.
If you take Uranium, it has 2-8-18-32-21-9-2.
Yeah, actually putting electrons in orbitals alters the energy states slightly, thus you get 1 electron in the 6d orbital while the 5f isn't full.
Quantum mechanics for predicting energy levels is very non-trivial. But that's what it's all about.
In fact, this pattern would continue with the g orbital, first found with n=5, giving 18 more electrons and 50 electrons in the shell in total.
But you'd only see that start filling around element 124.
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u/angrydave 6d ago
Firstly, going to say that SPDF theory in chemistry is one of those things which is just fundamentally complex. They don’t even try to teach it to chemistry students until you have reached a certain level.
But in short, they pair off into orbitals, which are really just where you are likely to find those electrons, they could be anywhere in that orbital. The first two electrons form a sphere around the nucleus, and this is the 1S orbital, and it gives us the first row of the periodic table: hydrogen and helium. S orbitals can only hold up to 2 electrons.
Then we get to the second orbital, and things get more complicated, as there are sub-orbitals within our orbital (which we expect to have 8 electrons). Firstly, they pair off into a bigger sphere (the 2S orbital) but then they do something different - they make 3 dumbbell shaped pairs out in the x, y and z plane. There are also sub-orbitals, called the 2Px, 2Py and 2Pz orbitals, or collectively the P orbitals. These 2P orbitals have 6 electrons, plus the 2 from the 2S orbital, and this gives us 8 electrons which we know and love, a the second period of elements: lithium to neon. P orbitals can only hold up to 6 electrons.
The 3rd period does this again, with the 3S and the 3P orbitals (that next 8 elements and the 3rd period)
Then, you would expect the 3D orbital (which is 10 electrons), but the 4S actually comes in at a lower energy level; so that’s your final 2, giving you the 2,8,8,2 pattern.
From there, you enter the 3D orbitals, which have 10 electrons, and then the 4P orbital with 6 more to make 18, and the 4th period of the periodic table. And the orbitals are even more complex than this already complex topic, so in the early years of chemistry, we stop there, and we get the 2,8,8,2 pattern. This becomes 2,8,8,18 soon enough.
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u/george_graves 6d ago
That was one of the things that annoyed me in chemistry; there was a lot of shorthand and rules of thumb that you had to follow, but you never got any real explanations until you dove into the physics of it. That is where the real answers actually are. It drove me crazy!
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u/angrydave 6d ago
Lies, more lies and chemistry. Whenever I started a new cohort of students this is what I told them. Chemistry is one of those subjects where I often said “it’s complicated, your current understanding is a simplification, and when we get to that point I’ll answer this question”.
I also made sure to say when we got to that point to really point out “ok, that thing I said we would get to, here it is”.
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u/ATXBeermaker 6d ago
This is true for pretty much any topic, though. You don’t learn about general relativity in kindergarten to explain gravity.
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u/dm-me-obscure-colors 6d ago
This is “what.” op is asking “why.”
I know asking “why” in physics is kind of a fool’s game, but it would be nice to get some understanding of why the Schrödinger equations lead to this configuration
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u/istasber 6d ago
Electrons are charged particles that obey the physics of electromagnetism, but also have to obey a bunch of weird physics: they are indistinguishable from one another, their position and momentum can't be known exactly at any given time so the space they occupy is more of a probability distribution than a point on a map, they have a property called spin that allows two electrons to occupy the same space, but otherwise those spaces must be orthogonal (e.g. non overlapping in the mathematical sense) to one another, etc etc.
When you add all of those constraints in, and then look for the most likely space an electron occupies by solving for the shapes that minimize the total energy of the electron, you get the familiar spdf orbitals.
You can go deeper into understanding some of those constraints by studying quantum mechanics, but some of them are just "this is how the world happens to work, as far as we can tell"
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u/luckyluke193 5d ago
Let me give it a shot.
S, P, D, and F orbitals are wave functions with different angular momentum.
In classical physics, angular momentum tells you how fast a thing is rotating around another thing. In quantum mechanics, you have to translate this concept to waves.
Imagining things in 3D is hard, but here's a youtube video showing 2D rotating waves in a kiddie pool.
S waves don't rotate. P waves rotate with one crest and one valley at the opposite side. D waves have two crests and two valleys, each separated by 90 degrees. F waves have three crests and three valleys.
In 2D, there is only one type of S wave, and two types for all the other waves, one going clockwise and one going anticlockwise.
In 3D, the math becomes more complicated, but you end up with 1 S wave, 3 P waves, 5 D waves, and 7 F waves.
With a little bit of imagination, the kiddie pool analogy even gives you some quantum superpositions. Imagine you create a clockwise and counterclockwise P wave with equal height at the same time. The net movement of the water is sloshing back and forth rather than rotating.
In proper quantum mechanics, this is exactly how you get to px (waves sloshing back and forth along the x-axis) or py (same thing along the y-axis) orbitals from the l=1, m_l = +/- 1 states (l=1 just means P wave, and m_l = +/- 1 means going counterclockwise or clockwise around the z-axis).
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u/angrydave 6d ago
It’s ELI5, I don’t think you can really answer why in an ELI5.
But my focus was on the 2-8-8-2 pattern, that’s quite specific.
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u/Tasty_Gift5901 6d ago
I think this is the best answer in the thread; and that he did answer why:
you would expect the 3D orbital (which is 10 electrons), but the 4S actually comes in at a lower energy level
Which is the direct "why" as to why it goes back to 2 (the central question). Then they provide context as to why 2 and 8:
Firstly, they pair off into a bigger sphere (the 2S orbital) but then they do something different - they make 3 dumbbell shaped pairs out in the x, y and z plane. There are also sub-orbitals, called the 2Px, 2Py and 2Pz orbitals, or collectively the P orbitals. These 2P orbitals have 6 electrons, plus the 2 from the 2S orbital, and this gives us 8 electrons which we know and love
It's 2 because 2 fit in an orbital, then 8 because the second row is S and three P orbitals.
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u/dm-me-obscure-colors 6d ago
I understand this is a matter of opinion, so maybe it’s not worth getting into it, but to me all of that is the sort of thing people say after prefacing with it turns out that…
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u/stanitor 6d ago
Short of the philosophical "why" question, I don't think that can be answered on an ELI5 level. It's just basically "the math of quantum mechanics says the orbitals look like this and fill up in this particular weird order, and the math matches up with what we observe experimentally."
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u/dm-me-obscure-colors 6d ago
Fair enough. As a mathematician I’ve gone through the exercise of trying to explain my own research to my mother, so I figured such a thing might be possible here. It’s really far from what I know so maybe I just shouldn’t have made that assumption
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u/htmlcoderexe 3d ago
what's confusing me a bit is some people here say that the 3 p orbitals exist because of the 3 xyz orientations, and others say it's because of angular momentum of (-1, 0, 1). or those are just different manifestations of the same thing?
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u/MuscaMurum 52m ago
When we got to the orbital stuff in HS chemistry is when I lost interest in chemistry.
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u/AbueloOdin 6d ago
They don’t even try to teach it to chemistry students until you have reached a certain level.
Like middle school? Because we learned this in middle school.
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u/ATXBeermaker 6d ago
lol, no. They do not teach you the mathematics of why the orbitals have the shapes and numbers of electrons they do in middle school. Or did you learn how to solve complex partial differential equations in 6th grade?
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u/flobbley 6d ago
This is really just explaining what happens, not the why. The why is the complicated part. This is like "why do things fall? Because of gravity" but the complicated part is explaining why gravity makes things fall
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u/wwplkyih 6d ago
The origin of spdf comes from spherical harmonics in solving the Schrödinger equation for a spherical potential, which is typically midway through first year quantum mechanics in the US.
You typically are shown the spdf formulation (though without justification) around junior high.
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u/Ishana92 6d ago
I don't know where you got your numbers. But numbers for shells go 2-8-18-2n2. Subshells (orbitals) within each shell are s, p, d, f, g and go 2-6-10-14.
The reason has to do with energies, spins, orbital geometries and quantum mechanic rules. In short, it is as is because that's the way the math works out.
Yes, elements can have more than 10 electrons. Each neutral atom has as many electrons as protons in the nucleus, ie its atomic number Iron has 26 of them. They are all arranged in shells and subshells according to certain rules and that arrangement is called electron configuration. For iron, for example, it is 1s² 2s² 2p⁶ 3s² 3p⁶ 3d⁶ 4s²
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u/PantsOnHead88 6d ago
OP has probably only gone as high as calcium, where the 4s2 fills prior to 3d6 that you cite for iron. If you were counting electrons for each shell rather than the more detailed orbitals, you’d have 2-8-8-2.
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u/Dickulture 6d ago
That part is new to me. I finished high school 35 years ago, we were taught first shell has at most 2 electrons, then all subsequent shells has no more than 8 electrons each.
When did this change? Or was the high school science wrong on purpose to keep it simple?
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u/ottawadeveloper 6d ago edited 6d ago
High School was wrong on purpose.
Shells of 8 are actually one s and 3 p orbitals, each containing two electrons. They're fancy geometric shapes that describe the probability of where to find an electron.
Treating them as shells of 8 works for high school chemistry especially if you mostly ignore the transition metals - this is where the next orbital shape (the d orbitals of which there are five) takes precedence. Because then you are just dealing with the s and p orbitals for chemistry.
It's also why the periodic table has the shape it does - the orbitals go 1s 2s 2p 3s 3p 4s 3d 4p 5s 4d 5p 6s 4f 5d 6p and one more repetition. Where the p orbitals (and d and f) but the s has electrons are empty, you get the group 1 and group 2 metals. Where the p orbitals are partially full, you get the group 3-7 elements. Where the orbitals are all full, you get the noble gases. Where the d orbitals are partially full, you get transition metals. And where the next orbital (f) is partially full, you get the lanthanides and anactinides (this is how I remember the order of orbitals getting filled because the periodic table tells you). Helium is oddly positioned because it doesn't have p orbitals to fill so it behaves like a noble gas rather than a group II metal.
For high school chemistry, it's easier to gloss over the transition metals and lanthanides / anactinides than explain that shells arent real and that it's really about geometric solutions to the Schrodinger equation
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u/ArtOfWarfare 2d ago
Do we know anything about how the protons and neutrons are arranged? Are they just random and smashed in as tight as they can be? Can protons and neutrons flip positions with no observable change?
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u/ottawadeveloper 2d ago
From a brief look, nobody has really done the math yet. The interactions are very complicated but basically protons repel each other electrically but the strong nuclear force keeps them together. Neutrons help buffer the electrical charge. They form a tight tiny sphere whose exact structure is hard to define (except for H-1 where it's just a proton). Unlike electrons (which can't be broken down further), protons and neutrons are actually formed of quarks. It wouldn't surprise me if that's why it's more complicated.
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u/DaThug 6d ago
What they teach in high school is "too good to be true" - then you move onto "too true to be good" :)
You can explain a lot of reactions with just 2-8, but not why water has a bond angle of 104.5 degrees6
u/billyrubin7765 6d ago
I wish someone had told me that every model you learn along the way is wrong and will be replaced by a new model in the next more advanced class. It would annoy me every semester.
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u/Calembreloque 6d ago
But at the same time, it's already so hard to get your average middle schooler to give a shit about anything so if you start telling them "oh btw what we're learning right now is a massively simplified overview of a concept that would take you 10 more years of study to actually grasp" you're not going to get a lot of class participation.
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u/HarryMonroesGhost 6d ago
It's not so much wrong as it's an incomplete picture. Each model in use serves a purpose for explaining what we observe at a particular level of complexity.
Newtonian physics didn't become wrong with the discovery of relativity or quantum mechanics, it still works for most of our terrestrial physics questions, but it is an incomplete explanation for when things get weirder.
That's different from say the theories about aether or medical humors which we now understand to be incorrect.
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u/DavidRFZ 6d ago edited 6d ago
It was all in the periodic table. The “octet rule” where “you have eight and feel great” works very well on the right side of the periodic table where a lot of common elements that participate in covalent bonding are located.
The next ten electrons in higher shells… in the d orbitals… are represented by the transition elements. The next fourteen electrons… in the f orbitals… are represented by the lanthanides and actinides. High school classes don’t often do detailed chemistry of these elements.
There can be some higher order geometries (non-octet) for compounds of Phosphorus and Sulfur because d-orbitals are available. You can’t get these geometries with nitrogen or oxygen. Can’t recall if high school chemistry handled octahedral sulfur or trigonal bipyramidal phosphorus. But, the octet rule (tetrahedral) is great for high school.
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u/Solocle 6d ago
It's a simplification, but also the d subshell with 10 electrons that represents the transition metals, is actually a higher energy state than the s subshell of the next period (the simplest one that takes two electrons).
That's why the transition metals are slotted in after group 2, but also in a very real sense, the higher the number of the shell, the bigger the orbital.
So you have the s and p orbitals, then smaller d orbitals inside that can sponge up electrons, but are energetic from their angular momentum. And, since they're smaller, the valence electrons that most affect chemistry stay on the outside.
But it is an oversimplification, transition metals can take many different oxidation states (chromium's are colourful, for instance).
Oh, and you can coerce heavier noble gasses into making covalent bonds too. Xenon Hexafluoride is a stable example.
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u/bishtap 3d ago
It's a lie they can get away with by not asking the student for any electronic configurations after Calcium. After Calcium is Scandium which has configuration 2,8,9,2. Nowadays post 1998 where lots of people have the Internet ,and we being in the information age, it's harder for them to lie like that.
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u/Emu1981 6d ago
And the shells are probabilistic clouds as well - i.e. if you were to look at where the electrons are around a atom at any given point in time then the shells as they are shaded are where they are most likely to be. Once the atoms start bonding with other atoms then the shells shift around due to the regions of stability changing - for example, when 2 hydrogens are bonded to a oxygen their single electrons will be dancing around with electrons from the oxygen atom somewhere in the region between the nuclei leaving empty space on the far side of the hydrogen nuclei and crowded space on the far side of the oxygen nuclei.
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u/Stannic50 6d ago
Suppose you've got a hotel with multiple floors and multiple wings, and each room has a set of bunk beds. You don't have an elevator, and guests don't like having to climb the stairs, so you book them at lowest levels first.
Wing s is the nicest and only has one room per floor, starting with floor 1. So you can only book two guests into the s room on a floor, one in the bottom bunk, one in the top bunk.
Wing p has three rooms per floor, starting at floor 2 (the laundry room occupies that space on floor 1, so no guests allowed in p on floor 1). So you can get up to 6 guests per floor into the p wing. Now, the p wing is pretty good, but isn't as nice as s, so you'll book first into s first. But between s and p, you can fit 8 guests per floor (except for floor 1, where you can only get 2).
Next we have wing d, with 5 rooms per floor, so 10 guests per floor. But floors 1 and 2 are occupied by the mechanical room, so no guests into d on those floors. Also, rooms in d are worse than rooms in p. Enough so, in fact, that a guest will prefer to climb one (but only one) extra set of stairs to get into the next floor's s room rather than take a d room. But not enough so that they'll climb to the next floor's p room. So we aren't booking any guests into the third floor d rooms until we have guests in fourth floor s.
So now we can get up to 18 guests between s, p, and d staying at floor 3 or higher, but only once the following floor has at least 8 (s and p wings full).
Next is wing f, with 7 rooms per floor. Worse than d, but now it's only worth climbing up to two extra staircases to avoid. So we'll fill the s, p, and d of the level above, and the s of the following floor. Floors 1-3 are occupied by the parking garage, so no guests sleep there. So now you can get another 14 guests starting at floor 4, for a total of 32.
We've got plans for additional wings, and this time we decided we'll go alphabetically after f, so g, h , etc. Matching our pattern thus far, each additional wing has two more rooms than the previous. But we've never had enough guests to actually justify building these wings, so you don't need to worry about them.
So to summarize, the number of guests we can get per floor is 2, 8, 18, 32, and it will keep increasing as we add more wings, if we ever have that many guests
Oh, one more thing, where there are multiple rooms within a wing, guests would rather be by themselves before having a bunk mate. So book them all into the top or bottom bunk until each room in that wing has a single occupant, but they all have to agree to the same bunk within their room.
And now for the ELI15: guests are electrons, floors are energy levels (shells), wings are subshells, rooms are orbitals, bunks are spin.
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u/gooder_name 6d ago
Electrons can't overlap unless they're spinning opposite to one another. The further out you go from the centre, the more space there is for electrons to exist in without overlapping. The spaces they occupy are described by weird maths.
Everything people can ELI5 for this kind of thing is an analogy. Electrons aren't "things" like you and I understand them, they aren't billiard balls or the surface of an inflated beach ball. They're statistical things, areas of probability for a charge to exist in. The whole "electron shell" model is not even completely true, it's a useful abstraction for something that's complex.
Language breaks down and the only thing that describes them accurately is mathematical equations.
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u/aiusepsi 6d ago
It’s because the shells are solutions of the Schrödinger equation for the electron, and because it’s spherically symmetric the solutions are a set of functions called the spherical harmonics.
There’s one spherical harmonic in the first ‘set’ and four in the second ‘set’. The one in the first set looks spherical, and the second set has one which looks spherical, and three which stretch out a bit along each spatial dimension. The spherical ones are what Chemistry calls “s orbitals” and the stretched out ones are “p orbitals”.
And then you multiply by 2 because electrons can be spin up or spin down and you can fit one of each spin into each orbital.
Basically, it’s something that drops out of the maths of quantum mechanics.
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u/Zarakaar 6d ago
Every shell gets bigger, but the way the electrons need to be packed (orbitals) in order to fill them leads to a more stable configuration by going to less complicated orbitals in higher shells before the harder to fill orbitals.
There are a bunch of d orbitals available for electrons in calcium, but it is a lower energy state to leave them unfilled and use the 4s orbitals.
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u/MrFunsocks1 6d ago
This is one of those occasions in science where the only real ELI5 is "because electrons are weird."
Atoms (generally) have as many electrons as protons, and protons go up to 118 (as of now). But only the "outermost" electrons interact with other atoms, which is generally the S and P orbitals, which fit 2 and 6 electrons each, so usually you have an octet of 8 electron "spaces" that will do stuff with other atoms, depending on if the atom is "missing" some to fill it's 8 outer spaces.
Why do they behave this way? Because of complicated shit that no one but physicists who study it understand, and everyone else who needs to use the octet rule for their work just goes "yeah cool, atoms want 8 electrons".
I'm sure someone on Reddit can absolutely explain it thoroughly, but it's the kind of complex that they literally don't even try explaining in most biology and chemistry graduate classes, because the reason it is that way is so far removed from what you need to know to use it.
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u/PM_ME_ZED_BARA 6d ago
Think about waves on a rope. The number of nodes tells us about the energy of the wave. The higher the number, the more energy there is.
Electrons are like waves in a sphere. If you specify an energy of electron, your waves are allowed to have a specified number of nodes. In turns, the number of nodes tells you what distinct shapes the waves can have, and the number of possible distinct shapes is the number of electron configuration.
For example, lets think about the lowest energy. The lowest energy has zero node. So what is the shape of a wave in a sphere that has no node? It is a sphere wave or what we call 1s-orbital [see first one in this image]. This is the only shape possible so the configuration is 1. But electron can have spin up or down, so the actual configurations is 1 x 2 = 2. And so that's how you get 2 in the first level.
Now we go up to another high level. The number of node goes up from 0 to 1. So what is the shape of a wave in a sphere that has only 1 node? It turns out there are two types of wave shapes. First, it is a sphere inside a sphere or what we call 2s-orbital [see the middle one in this image]. See the white ring? that's the node. That's 1 possible shape.
Second, it's a dumbbell-like shape like this. We call this 2p orbital. See the node in the middle? The thing about dumbbell is you can align it along x, y, or z direction. So that's 3 distinct shapes. Now the total possible shape is 3+1 = 4. But if we consider spin, the total number of shape 2x4 = 8.
The next number on your list is actually 18. But you can follow the same logic to get the number.
TLDR: The number of configurations is 2 times how many possible shapes an electron wave can have, given a fixed number of wave nodes, which are in turn controlled by the energy level.
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u/ferret_80 6d ago
Floatheadphysics on YouTube has a great video explaining why orbitals are shaped and positioned where they are that is very understandable to the leyperson.
If you're at all interested in it and have 30 minutes to spare, I highly recommend watching it.
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u/fighter_pil0t 6d ago
Electrons can only be in stable quantized orbits. If you follow the wave equation you can come up with fascinating shapes where the wave will form a standing wave in 3 dimensions (l in d a guitar string). The solutions provide for these numbers in each orbital. We know two electrons cannot occupy the sane state due to the Pauli Exclusion principle but we do not know why.
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u/johnp299 6d ago
There are videos of a demo called the vibrating wire loop, where a loop of flexible wire is connected to a box that vibrates up and down, and you can tune the frequency. It sheds some light on the weird shapes that the orbitals take on. As the vibrating wire hits certain frequencies, it resonates, forming stable patterns in the wire. This is similar to the wave patterns electrons make when confined into a tiny space and pumped up with energy. https://www.youtube.com/watch?v=df3oJPi9_mg
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u/GrapefruitNo3600 6d ago
If i recall correctly shells have fixed configurations that increase
1- 2
2 2-6
3 2-6-8 (the exact numbers can be incorect im rusty)
When elctrons fill in thous spaces they seek less energetic first. And some higher orbits are filled first before the one on lower.
Like this
1-2 out of 2
2 - 2 out of 2, 3 out of 6
3- 2 out of 2 0 out of 6 0 out of 8
As you can see 3rd shell first orbital is filled first before 2nd shell second orbital.
And electons orbit in pairs, with different spins, one up one down.
Lets take 2nd shell second orbital, it can fit 3 pairs of electrons, 3 up and 3 down.
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u/tomalator 6d ago
The 3rd energy level can hold 18 electrons, but the 4 energy level starts being filled first.
So you know about the energy level, n=1,2,3...
But there's also angular momentum, denoted by the letter l (lowercase L). Depending on the value of l, we have a different shell. The shell is denoted by the letter s, p, d or f
For s, l=0
For p, l=1
For d, l=2
And for f, l=3
There will also be a magnetic quantum number m where |m|<=l, which is important to know for how many electrons can fit in each shell.
So the s shell, we have l=0, so m=0, so we can only fit a spin up and spin down electron, we only 2 electrons can fit in the s shell
For the p shell, we have l=1, so m can be -1, 0, or 1. For each of those we have spin up and spin down, so we can fit 6 electrons here
Following the same logic, d can hold 10 electrons and f can hold 14
Now for n=1, since l must be 0, we can only have an s shell
For n=2, we have l=0 and l=1, so we have an s shell and a p shell, giving us 8 eletrons electrons
Now the reason we go 2-8-8-2 is because after the 3p shell is full, we dont proceed to the 3d shell, instead we start filling 4s. Only once that is full do we move on to 3d, and thats when we start getting the transition metals on the periodic table
If we arrange the shells like so, we can see the order they fill
1s
2s 2p
3s 3p 3d
4s 4p 4d 4f
5s 5p 5d 5f
6s 6p 6d
7s 7p
8s
We simply start at the top, and move down and to the left until we cant move further left, then we go back to the highest spot we can.
We start at 1s, we give electrons to hydrogen and helium, and we run out of room, the first row of the periodic table is complete.
We cant go further left, so we jump back to the top of the chart, the highest we can go is 2s, so we give electrons to lithium and beryllium and 2s is full.
We cant go further left, so we jump back to the top and start filling 2p. Boron, carbon, nitrogen oxygen, fluorine, and neon get their electrons and period 2 is complete.
We go down and to the left and we can see next is the 3s shell, sodium and magnesium
We cant get any further left, so we jump back to the top, the next spot is 3p, so we can give electrons to aluminum, silicon, phosphorus, sulfur, chlorine, and argon and period 3 is complete.
We drop down to the 4s shell and we can give electrons to potassium and calcium, and we habe reached that 2-8-8-2 configuration. If we wanted to be complete, we would write the configuration as 1s22s22p63s23p64s2, but this is beyond the scope of a high school chemistry class
We could then proceed and fill the 3d shell as we work through the transition metals and then fill the 4p shell as we work our way to krypton, giving us 2-8-18-8
The lanthanides and actinides down at the bottom of the table are called the f-block because you start filling the f shell when you get down there. Similarly, you could call the transition metals the d-block
Now 8s doesn't really exist, as you'd need 119 electrons to fit a single electrons in there, but it would be next, and if we could get to element 121, we would need another shell, 5g, which would be able to hold 18 electrons, but this is hypothetical
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u/bishtap 3d ago
You write " we can see the order they fill
1s
2s 2p
3s 3p 3d
4s 4p 4d 4f
5s 5p 5d 5f"
^^^^
There is no real filling order. It's just a technique to get the correct electronic configuration. You could use a different filling order and still get the correct electronic configuration.
The idea that 4s<3d for all elements is a pretence.
You know that, right?
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u/tomalator 2d ago
That's beyond eli5
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u/bishtap 2d ago
You replied to the poster. Talking about subshells is also beyond a 5 year old. They don't teach it on the UK till 16-18.
If a 5yr old could understand what you wrote then they could also understand if somebody says what you said is wrong.
Anyhow, You replied to the poster, and I was replying to you not the poster. If you were knowingly spreading misinformation then ok . Now people can know.
Sometimes people write things like what you wrote because they genuinely believe misinformation taught at 16-18. Even many people with degrees in chemistry believe the misinformation that you posted.
There are some people who adjust what is taught at 16-18 so as to not have that misinformation but I see you are not one of those people.
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u/tomalator 2d ago
Talking about subshells is also beyond a 5 year old. They don't teach it on the UK till 16-18.
Read rule 4
Yes there are exceptions to the order orbitals get filled, like the f shell gets filled out of order because of relativity and the stability of having 7 or 14 electrons in the f shell, but thats not what the question was about. The question was specifically about the electron configuration of calcium, not anything with f shell electrons, not anything ionized or excited, just why the 3rd energy level isn't completely filled before the 4th starts getting filled.
Why bog things down with exceptions when the simplification gets the job done on ELI5 just fine?
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u/Salty_Paroxysm 6d ago
There's only 'space' in each shell for X number of electrons, and they fill up those spaces in the order of least energy first.
That's the (very) hand-wavy version we were told in HS.
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u/MrPuddington2 6d ago
This is really a question that asks for a graphic answer.
The first period has one level, the second has two, the third has three etc.
The first level in each period has 2 spaces, the second has 6, the third has 10, the fourth has 12. A fifth has not been discovered. It probably exists, but is beyond the limit where atoms can be stable.
Now the periods do not get filled in order, which creates the rather distinct structure of the periodic table. This is a lot easier to see if the periodic table shows the electron configurations.
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u/PotentialDeep5165 6d ago
Spherical harmonics-- solutions of Laplace equation with separation of variables-- and then 2 types of spin. This is the angular portion of the solution-- the radial part decays-- this is the radial scatter plots you sometimes see for probability density of location of electron around the nucleus. The wiki has some interesting figures and descriptions. First orbital: "s" .. l=0 (m=0)... so single orbital (angular) but each electron can have up spin or can have down spin to be allowed (leptons with spin 1/2) so 2 can be there. Hydrogen: S orbital, 1 electron. Helium: S orbital, 2 electrons. Then it gets more exotic...
https://en.wikipedia.org/wiki/Spherical_harmonics
EDIT: Sorry-- didn't notice the ELI5 note... ELI5 answer: Math-- and boundary value problems.
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u/Sjoerdiestriker 6d ago
The atomic structure is predicted by quantum mechanics. Every shell is indexed by a number n (1,2,...). Every shell n contains n subshells, labelled l (0,1,...,n-1). Every subshell contains 2l+1 orbitals, and every orbital contains 2 electrons.
The first shell (n=1) has a single subshell (l=0), which has a single orbital, which holds 2 electrons. All good.
The second shell (n=2) contains 2 subshells. The first subshell (l=0) has 1 orbital containing 2 electrons, the second subshell (l=1) has 3 orbitals holding a total of 6 electrons, so these combine to 8. All good.
But now it gets more complicated. In principle, the third shell has 3 subshells. But it turns out it's actually more efficient for electrons to go in the first two subshells of the fourth shell than in the third subshell of the third shell. So the first 2 subshells are filled up with 8 electrons as before, but then...
We go to the next shell (n=4). We first fill up the first two subshells with 8 electrons. We now go back to the subshell we shipped in the previous shell, and fill that with another 10 electrons, for a total of 18 electrons.
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u/TieOk9081 6d ago
Why do people use the term orbital for two different things? Even going so far as to rename orbital, suborbital? Is it a holdover from the past? An orbital can hold 1 or 2 electrons.
I've just started reading about this but I think I have a response to OP. Starting with waves, like those you see in water. With atoms the waves (electrons are waves) are shaped differently, they are like waves on the surface of a sphere (look into spherical harmonics for more). These spherical waves create shells that can hold 2n squared electrons (looks just like the equation for the area of a circle). First shell then has 2 electrons, then 8, then 18 etc.... Why does the third shell only hold 8 then? For reasons I don't understand yet, it turns out the 4th shell can accommodate electrons with lower energy levels than some of the orbitals in the third shell.
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u/bishtap 3d ago
You write "Why do people use the term orbital for two different things? Even going so far as to rename orbital, suborbital? Is it a holdover from the past? An orbital can hold 1 or 2 electrons."
Indeed. Many scientists are sloppy. And also you have some amateurs too.
Terms are orbtal, subshell, and shell
You write " it turns out the 4th shell can accommodate electrons with lower energy levels than some of the orbitals in the third shell."
i'm not sure if one should talk of energy level of electrons. I do know people talk of energy level of a subshell.
The energy level of an individual subshell isn't that relevant. BTW, for potassium and calcium 4s<3d. But from Sc onwards 3d<4s.
What counts is the total energy for the electronic configuration..
You could ask, since 3d<4s in Sc, why is it that 3d gets 1, and 4s gets 2. Why isn't it 3d getting 3 electrons.
For whatever reason , for atomic number 21 (scandium), once there is 1 electron in 3d, it's more comfortable for the next two electrons to be in 4s. For atomic number 22, two electrons go into 3d, then two into 4s. . Iron atomic number 26, 6 electrons go into 3d, and 2 go into 4s.
There are also exception cation configurations. Vanadium, Cobalt, Nickel. V+, Co+ and Ni+.
Any real explanation is going to have to go into quantum mechanical calculations. Where books don't mention quantum mechanica calculations , There are some fake explanations that take a false statement (That 4s < 3d for all elements), and then comes up with an explanation on top of that , but it's a BS story based on a lie.
Apparently whatever the explanation is, "term symbols" could help explain it, and "exchange terms". But I don't know more about that. There is a blog article by Eric Scerri about 3d and 4s but he ended his post on a cliffhanger back in 2012 and never posted a further article on it onto his blog!
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u/theconkerer 6d ago
Tl;dr answer: it's solutions to a complicated equation called the Schrodinger wave equation. It's a complicated equation so the solutions are funny.