Transition metals sit in the central block of the periodic table. Typical properties: high melting points, high density, good conductors of heat and electricity, and they form coloured compounds. Many are useful catalysts (iron in the Haber process, nickel in hydrogenation).
Compared with Group 1 metals, transition metals are harder, stronger, denser and much less reactive — they react slowly or not at all with water and oxygen.
An alloy is a mixture of a metal with at least one other element. The different-sized atoms disrupt the regular layers, so the layers cannot slide over each other easily — this makes alloys harder than the pure metal.
Examples: steel (iron + carbon), stainless steel (iron + chromium + nickel, resists corrosion), brass (copper + zinc), bronze (copper + tin).