Physical properties and trends

Almost every property of the transition metals comes back to their d electrons. Draw the five d boxes and fill them one electron per box before pairing: iron's 3d⁶, for example, leaves four unpaired electrons.

Unpaired d electrons join in metallic bonding. The more there are, the stronger the bonding. That explains a whole chain of properties:

• Transition metals are hard and have high melting and boiling points.
• They have a high enthalpy of atomisation: it takes a lot of energy to pull the atoms apart.
• The values peak in the middle of each series, where unpaired electrons are most numerous.

Zinc, cadmium and mercury sit at the other end. Their d¹⁰ electrons are all paired and take no part in metallic bonding, so these metals are soft and melt easily. Zinc has the lowest enthalpy of atomisation in its series, and mercury is a liquid.

Size: across a series the atoms get slowly smaller. Each new electron goes into an inner 3d orbital, which shields the outer 4s electrons only partly from the growing nuclear charge.

Ionisation enthalpy rises across a series, but slowly and unevenly. It dips or jumps where a stable d⁵ or d¹⁰ arrangement is made or broken.

Going down a group, the 4d element is bigger than the 3d one, as expected. But the 4d and 5d elements of a group are almost the same size: zirconium and hafnium, for instance. The reason is the lanthanoid contraction (see the lesson on the f-block).