Who developed the periodic table's ordering by atomic mass in 1869?

Mendeleev, and what made his table matter was not the ordering but the gaps. Others had noticed periodic patterns; he was willing to leave holes where no element was known, predict the properties of what belonged in them, and stake his reputation on it - specifying the density, atomic weight and even the melting behaviour of substances nobody had seen.

Gallium, scandium and germanium were then found within fifteen years, matching the predictions closely enough that argument stopped. He was also prepared to override his own organising principle: where atomic mass put an element in a chemically wrong place, he swapped them, which turned out to be right for a reason he could not know. The true ordering is by atomic number - the count of protons - which was not understood until Moseley's work in 1913.

He never won a Nobel Prize, missing by one vote in 1906 and dying the following year, apparently blocked by a committee member with a grudge over a different dispute. He also worked on standardising Russian vodka, which is often exaggerated into his having invented the 40 per cent standard - and element 101 is named mendelevium, which is a more durable memorial than a prize.