Why is the position of hydrogen in the periodic table anomalous?

Hydrogen has one electron in its outermost shell like the alkali metals, and forms H⁺ and a unipositive ion as they do. But it is one electron short of a stable shell like the halogens, forms a diatomic molecule and a hydride ion H⁻ as they do, and it is a non-metallic gas. It therefore resembles both groups and belongs completely to neither.

No citable source has been recorded for this record. Treat it as practice material, not as fact.

Explanation

It is usually placed at the head of group 1 for convenience.