d-block

The d-block corresponds to the transition metals of each period — up to ten per period — where the (n−1)d subshell is filled in an often non-monotonic way (exceptions at Cr, Cu, Mo, Ag…). These elements share striking features: multiple stable oxidation states, coloured coordination complexes, industrial catalysis, magnetism. The d-block holds the world's most-used metals (iron, copper, nickel, zinc) and precious metals (gold, platinum, rhodium, iridium). The filling rules are subtle because the ns and (n−1)d subshells are very close in energy.