anomalous position of hydrogen
Where do you seat hydrogen at the periodic table's dinner party? It is the very first element, just one proton and one electron, and that simplicity turns out to be a problem: hydrogen behaves a little like the alkali metals on the far left, a little like the halogens on the far right, and not quite fully like either. Most printed tables park it above lithium in Group 1, but that is a convenience more than a verdict, and you will sometimes see it floating alone above the whole table.
The reason for the ambiguity is its single electron in a 1s orbital. Like a Group 1 metal, hydrogen has one electron to lose and forms a +1 cation — but H+ is a bare proton, vanishingly small and never found naked in chemistry; it is always grabbed by something else, as in the hydronium ion H3O+. Like a halogen, hydrogen is one electron short of a noble-gas (helium) configuration and can gain an electron to form the hydride ion H- — but H- is large, soft, and a powerful reducing agent, quite unlike a true halide. And unlike either family, hydrogen mostly prefers to share its electron in covalent bonds, as in H2, water, and the millions of organic and inorganic molecules built around it.
The honest takeaway is that hydrogen is a one-of-a-kind element that does not truly belong to any group; its placement is a labeling compromise, not a statement about deep similarity. This matters because hydrogen is the most abundant element in the universe and a thread that runs through nearly all of chemistry — acids and bases, water, hydrides, the hydrogen bond, fuels — so understanding why it resists pigeonholing keeps you from over-trusting the table's tidy columns.
Hydrogen forms H+ in acids (like a Group 1 metal would shed an electron), H- in NaH (like a halogen would gain one to reach a noble-gas shell), and shares electrons in H2 and CH4 — three faces, no single home.
The same element acting metal-like, halogen-like, and uniquely covalent.
Putting hydrogen above lithium does not mean it is an alkali metal — it is a gas, not a soft reactive solid, and forms a tiny bare cation rather than a metal lattice. The placement is convention, not chemistry.