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Q & A: Atom's Shells and Orbitals

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Q:
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
A:
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. Itís 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 hereís the basic idea for orbitals the way that youíre 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 donít 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 wonít 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 havenít 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 youíre 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 Periodic Table.) 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 isnít 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 youíd 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 youíd think. For example, letís 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 Calciumís 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 thereís 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

(republished on 07/21/06)

Follow-Up #1: atomic shells

Q:
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
A:
Youíre on the right track. The two 2S electrons share exactly the same íorbití but have opposite spins, so they arenít in the same exact quantum state. The part thatís a little different from your picture is that the states these electrons are in arenít 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 íorbití, i.e. the same shape of cloud. The clouds in this case arenít spherical but rather have lobes. The three different 2P orbital states differ in that their lobes point different directions.

Mike W.

(published on 08/16/07)

Follow-Up #2: electrons and energy

Q:
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 donít know if my question makes sense... But please do reply me, thanks.
- Emma
Canada
A:
Hi Emma- I sort of can guess what youíre asking. The electrons in one shell all have about the same energy. Since the low energy shells fill up first, at least when things arenít 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 08/21/07)

Follow-Up #3: orbital existentials

Q:
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
A:
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 03/02/09)

Follow-Up #4: orbital misery

Q:
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
A:

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: http://winter.group.shef.ac.uk/orbitron/.  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 09/25/13)

Follow-up on this answer.