Where do hydrogen and helium really belong in the periodic table?

Hydrogen and helium do not fit cleanly anywhere, and that is why chemists still argue about them. Although the modern periodic table is standard, the placement of the period 1 elements, hydrogen and helium, remains an open issue under discussion, and some variation can be found. In practice hydrogen usually sits in group 1 and helium almost always in group 18, but each placement is a compromise.

By electron configuration alone, hydrogen would go in group 1 and helium in group 2. Hydrogen in group 1 is common, since like the group 1 metals it has one outer electron and typically loses it in reactions. But hydrogen also forms hydrides by gaining an electron, as halogens do, and the lightest halogens are gases like hydrogen. It matches neither group perfectly, and is difficult to place by its chemistry. Rarer layouts put it in group 17, in both groups 1 and 17, or floating outside all groups.

Helium is almost always placed in group 18 with the noble gases, because it is unreactive and has a full outer shell. But it has two outer electrons while the other noble gases have eight, and it is an s-block element while they are p-block. Solid helium crystallises like beryllium and magnesium in group 2. IUPAC rejected a proposal to move helium to group 2 in 1988, but recent theory suggesting that helium could form a compound like beryllium's has led more chemists to advocate placing it there.