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!
(republished on 07/21/06)