[Sophia7X]

How come aluminum's oxidation number is +3 and thallium's (stable) oxidation number is +1, and they both are in group 3A? It seems like thallium should have the higher oxidation state because it's bigger than aluminum, so wouldn't it be easier to ionize?

[Schrödinger]

google : Inert pair effect

[AWK]

Al also form AL(I) compounds though very unstable (W. Klemm 1948).

[Hunter2]

This behaviour you can see also in other group in the periodic system. 3. group light elements +3 B, Al, Ga , heavy ones +1 In,Tl, 4. group light elements +4 C,Si, Ge  heavy ones +2 Sn,Pb, 5. group has both, +3 and +5 N,P,As,Sb, heavy ones +3 Bi 6. group light ones O -2  , only with flour +2, heavy one +6 , S, Se, Te, heavy one  +2 Po. 7. group -1 F, other until +7 Cl, Br, I

[AWK]

6. group light ones O -2  , only with flour +2

KO₂ and KO₃ really exist!

[Hunter2]

I agree, also H₂O₂ exist.