Electron Areangement

Question

They gave two example:

Oxygen’s electron arrangement is 2-6 and oxide ion is 2-8. 

How do I find that out? 

Sodium and sodium ion

Aluminum and aluminum ion

Answer 1

O is in period 2 group 16 so it has 2s2 2p6 or 2-6. The oxide has 2 additional electrons (O^2-) so would be 2-8.

Na is in period 3 group 1 so it has 3s1 or 1. The sodium ion (Na⁺) has lost that one electron to become isoelectronic with neon (Ne).

Al is in period 3 group 13 so it has 3s2 3p1 or 2-1. The Al ion (Al³⁺) has lost those 3 electrons to become isoelectronic with neon (Ne).