Why can the first shell of an atom only contain 2 electrons?

Answer 1

The answer to this is complicated, but involves the symmetry of the orbitals (roughly visualized in terms of their shape, although "shape" doesn't have a strict meaning in a quantum mechanical framework (damn that Heisenberg!)) and the fact that electrons are fermions. What I am about to say next would have a real quantum chemist beat me to death, so as a disclaimer I state I am not a quantum chemist. I tried to be one once, but, well, that's a long story. But I digress.

Anyway, the orbitals with the minimum energy, the first shell, are spherically symmetric. That means once you put an electron in one, it occupies the space around the nucleus uniformly. Since the first electron doesn't have a preference for location, the only way to put another electron in the level is to flip its spin. But spin can only be up or down, so only two electrons can go into the lowest, spherically symmetric, orbitals.

In contrast, the p orbitals, do have a preferential orientation (not really, what they have is angular momentum, but we kind of visualize this as a shape, sort of (this is where the real quantum chemist is about to core me a new one)). Because they have a preferential location, you can put another electron in at that same energy without flipping its spin since it will occupy a different location around the nucleus (not really, it has a different angular momentum ... oh never mind). Then, you can still do the spin-flipping trick to put two electrons into each of the p orbitals.

The cool, or boring, thing is that once you have filled the p orbitals, the resulting distribution is spherically symmetric, and that makes the energy level happy so it compresses a little. This is sort of why noble gases are noble, and why things want to get to be noble-like, they are spherically symmetry in terms of the orbital occupation.

This same concept could be applied to d orbitals, and f, that the spatial sum of their PDFs adds to be spherically symmetric (I think it's more complicated than that, but after drinking heavily I am too wasted to look it up to be sure).

Anyway, after typing the above crap, for lack of a better term, I did check to see if the wikipedia entry was at least vaguely similar. Happily, it is, sort of. So below is the wiki on orbitals.

Answer 2

The first shell only contains 2 elections due to orbitals.

Answer 3

Because there is only 1 orbital in this level - an s-type - and it can only hold two electrons.

Answer 4

2, if by orbital you mean fully specified n, el, m(el). If that's not what you mean, refer back to the rules for possible values of el and m(el). The answer works out as a total of 2n^2 if n is the principle quantum number.

Answer 5

its just the way it is man, why does 2 + 2 = 4?
something things in life can not be explained, and this my friend you have you remember