Question

How do we conceive that elements typically fill up their orbitals?

Answer 1

Certain elements can only have 8 electrons in their outer shell.

Explanation:

This happens only with period 2 elements. Hydrogen and helium can only get a maximum of 2.

I also said 'certain elements' as a lot of elements on period 3 onwards, especially on the RHS of the table can use d-orbitals to have more electrons.

For period 2 elements, elements can only have 8 electrons in their outer energy level before it moves to an s-shell of a higher energy level. i.e. argon is `1"s"^(2)2"s"^(2)2"p"^(6)3"s"^(2)3"p"^6`, if you look at the 2nd energy level, you'll see that there are 8 orbitals in total.

The s-subshell can only hold 2, the p-subshell can only hold 6, which leaves 8 in total. Elements which can access the 3rd energy level, have access to d-orbitals which is what allows some elements to expand their octet.

As a GENERAL guideline, one may predict the following filling order (which is subject to many exceptions!):

`cancel(1s)`
`cancel(2s), cancel(2p)`
`cancel(3s), cancel(3p)`
`vdots " "cdots`

If you follow the lines diagonally you will find they go in the order: 1s, 2s, 2p, 3s, 3p, etc. Past that, it's up to the atom what the orbital ordering is.

You can see that from period 3 onwards, elements have a d-orbital available. s-subshells can only contain 2 electrons, p-subshells can only contain 6 electrons, elements which only have s- and p-orbitals can only have 8 electrons in their outer shell (2+6=8). However d-orbitals allow for a maximum of 10 extra electrons.

These elements can use the d-orbitals to expand their octets due to the same principal quantum number `n` (www.thoughtco.com/definition-of-quantum-number-604629).