How do I know the likely oxidation states of metals, and non-metals?
Well main group metal ions typically form an ion whose charge is equal to their Group number............i.e. isoelectronic with the last Noble Gas.
........And non-metals typically form an ion isoelectronic with the next Noble Gas.
A Group 17 element typically forms an ion with a single negative charge:
And a Group 16 element typically forms an ion with a double negative charge:
In each case the element has formed an ion isoelectronic with the next (or last) Noble Gas. And we can go even farther than this, and consider Group 15.
And we can look at oxidation of the alkali metals (Group I):
And of the alkaline earths (Group 2):
The point is that the Group number reflects electronic structure, i.e. the number of electrons present in the valence shell. Group I and Group II metals have 1 and 2 valence electrons respectively. As atomic number increases across a Period, nuclear charge increases accordingly, and non-metals, to the RIGHT of the Period as we face it, tend to be oxidizing, and ADD electrons to their valence shell.