Q & A: Atom's Shells and Orbitals

how do you determine how many shells an atom has and how many electron do those shells contain?
- Anonymous (age 14)
L.A. school district

Good question! This is actually a very complicated question, and it's important to understand that orbitals and shells are basically just a good way to /think/ about what happens in the atom. It’s a bit truer to say that the electrons just buzz around in clouds around the nucleus, but this is definitely harder to think about. But here’s the basic idea for orbitals the way that you’re probably learning about it in school:

Every atom basically has an infinite number of shells. The thing is that almost all of those shells are empty (they don’t have electrons in them). Electrons generally go into the orbital with the "lowest energy."

The first orbital that fills up is called the 1S orbital. This one can hold 2 electrons. Once you get to the third electron, you have to put it in the next orbital. This one is called the 2S orbital. (All "S" orbitals hold 2 electrons.) Next comes the 2P shell. "P" orbitals hold 6 electrons. After 2P is 3S, 3P, 4S, 3D, 4P, etc. ("D" orbitals hold 10 electrons each.) The order gets pretty complicated once you get this far, but you probably won’t need to know them much past 3P.

If your teacher is trying to make this less confusing, they may just pay attention to the numbers, not the letters. In this case you can just add them up. Shell 1 (1S) holds 2 electrons. Shell 2 (2S & 2P) and Shell 3 (3S & 3P) each hold 8 electrons. After that, the next 2 hold 18 each, then the next 2 hold 32. So far, scientists haven’t discovered any elements that use more orbitals than this.

As for "how many shells the atom has," I already said that it has an infinite number of empty ones. But the number of full ones is probably closer to what you’re looking for. For example, lets look at Calcium. Calcium has 20 electrons. (You can see this by looking at the atomic number of Ca on a .) The first 2 go in shell 1, leaving 18 more. 8 more go into shell 2, then 8 in shell 3. Then there are 2 left. This isn’t enough to fill the next shell, which would require 18 more, so we say that Calcium has 3 filled orbitals and 2 free electrons.

If an atom is what is called an "ion," then that means it has a different number of electrons than you’d expect. This is written by using little numbers with + or - after the atom's letters. Because electrons actually have a negative charge, a - sign means that the atom has more electrons than you’d think. For example, let’s take Calcium again. Like we said, Ca has 20 electrons. But Ca- has 21, and Ca2- has 22. Going the opposite way, Ca+ has 19, and Ca2+ has 18. (Since 18 makes exactly 3 filled orbitals, Ca2+ is Calcium’s most common ion.)

Ok...going back to what I said about electrons going to the orbital with the "lowest energy." What if you were to give those electrons some more energy? Lets say we take Helium, which has 2 electrons. These 2 electrons will be just enough to fill Shell 1. If I give them some more energy, one of those electrons jumps up to Shell 2. So there’s 1 electron in Shell 1 and 1 in Shell 2. But as soon as it can, this electron will fall back to Shell 1 again. When it does, it will release energy as light.

Hope this answers your question!

-Tamara

(published on 10/22/2007)

I am lost when you say like 2S and 2P and I know that they are called subshells. But what does subshell really mean? Does it mean that eg. in 2 shell, 2 electrons would be together spinning in an orbit and 6 other electrons would be doing so in other orbit of same energy level? I really want to know and thank you.
- Thurein (age 17)
Kingston, Jamaica

You’re on the right track. The two 2S electrons share exactly the same ’orbit’ but have opposite spins, so they aren’t in the same exact quantum state. The part that’s a little different from your picture is that the states these electrons are in aren’t anything like classical orbits. Rather, the state is a wavelike cloud spread around the nucleus. For S-type states, the cloud has a perfectly spherical shape.
For the 6 2P electrons, there are as you say 3 opposite-spin pairs of electrons, with the two electrons in each pair having the same ’orbit’, i.e. the same shape of cloud. The clouds in this case aren’t spherical but rather have lobes. The three different 2P orbital states differ in that their lobes point different directions.

Mike W.

(published on 10/22/2007)

Based on the first answer, about adding energy to one of the electron in Shell 1 and making the electron to jump to shell 2..... Did you mean that all electrons has a specific amount of energy in them? So they would be classified into different energy levels in an atom?
I don’t know if my question makes sense... But please do reply me, thanks.
- Emma
Canada

Hi Emma- I sort of can guess what you’re asking. The electrons in one shell all have about the same energy. Since the low energy shells fill up first, at least when things aren’t too hot, it does take a specific amount of energy to raise an electron from some particular shell to another, higher energy shell.  In real atoms, there are some complications because the energy depends not only on the individual electron but also on all the other electrons.

Mike W.

(published on 10/22/2007)

Lets say i have a single electron in the 1s orbital and i zap some energy in and make that electron go to n=2 does the 1s orbital dissapear ??? or is it filled with some other kind of energy?
- Azeem Notta (age 19)
Toronto, Canada

Your question helps clear up a common misconception. The orbital isn't something sitting there, which may or may not have an electron in it. The orbitals are more of a list of different ways electrons can behave. If the single electron is doing its 2S thing, then it's not doing its 1S thing. That's all there is to it.

Mike W.

thanks, Inga

(published on 05/16/2013)

I Still Don't Get What ANY Of This Orbital Stuff Means.... This Is So Confusing.... I Hate Science It Should Kill Itself!
- Devin (age 14)
Clovis CA

Yeah, that orbital stuff is complicated. I'm not sure why it gets introduced so soon in school. There would be a lot of much clearer things to learn.

Just for a little start on orbitals, since something about them was probably assigned, you might look at these pictures of some different orbital shapes that the electron cloud can take: .  Orbitals have stable shapes that don't change in time. Other states do change in time.

As for your wishes for science, perhaps providing governments with nuclear weapons was a step in that direction. If they are used, however, there will be collateral damage.

Mike W.

 

(published on 09/19/2013)

Most of what you have said makes perfect sense However I am confused on how there can be more than two electrons in a shell. You see I am directly speaking about the Pauli exclusion principle that states that two electrons in the same atom can't be in the same quantum state. This is like all fermions. Now because there are 2 spins this will account for two electrons in a shell however more doesn't make any sense. Care to explain what is really happening?
- Taylor (age 14)
UK

If each shell represented a spherically symmetrical cloud form ("S state"), you'd be right that each would hold only two electrons, given the two spin states. However, there are states that are not spherically symmetric. One way of expressing the P states has two lobes of cloud, with opposite sign of wave in each lobe. There are three independent ways of aligning those lobes, e.g. along x,y, or z  axes. By symmetry these all have the same energy. For D states there are more lobes and five independent forms. In the conventional shell language, states that have almost the same energy, such as the lowest energy P states and the second-lowest-energy S states are lumped into the same shell, giving even more states per shell.

Mike W.

(published on 07/06/2016)