Question

Two of the energy levels can hold eight electrons each. Which energy levels are these?

Answer 1

Actually, it is only the second shell that can hold eight electrons, as it consists of only one `2s` and three `2p` orbitals.
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Explanation:

It is often claimed that the third shell will hold only eight electrons as well, but this is not correct.

This claim is made because when one looks at the periodic table, one finds eight elements in the second row, corresponding perfectly to the eight valence electrons that will occupy the second shell (or energy level, as it is sometimes called).

Then, to account for why the third row also has eight elements, we state that the third shell only holds eight electrons as well.

In fact, things are a bit more complicated than that! What actually happens is that the third shell does acquire eight electrons (in the `3s` and `3p` orbitals) as the valence shell, but this does not fill it, as it still has a group of five orbitals known as `3d`, which can accommodate another ten electrons. (Every orbital holds up to two electrons, regardless of energy level.)

The complication comes in because the fourth shell (specifically the `4s` orbital) fills before these `3d` orbital do. So, the fourth shell has begun, and is the current valence shell, before the third shell is complete.

The ordering occurs in this way because the electrons are populating orbitals at the lowest possible energy level, which for atoms is not quite the same as the order based on distance from the nucleus. (But that might be a story for another time!)

Now, here is the cool part - the ten electrons of the `3d` orbitals fill next, starting with element 21 (scandium) and the block of elements known as the transition metals are underway!

So, while it does get a bit complex, the correct ordering of the electrons as they populate the energy levels of an atom does correctly account for the layout of the periodic table.