Explain, with reasons, the trends in (i) melting point, (ii) atomic and ionic size and (iii) ionisation enthalpy across the 3d series.

How to lay out the answer. Follow the frame, then write it in your own words.

1 · Open with the key idea. Across the series the nuclear charge rises and electrons enter the inner (n−1)d orbitals.
2 · Melting point. Rises to a maximum near d⁵, then falls; d and s electrons both take part in metallic bonding. Zn, Cd, Hg (d¹⁰) are low melting.
3 · Size. Ionic radius (same charge) decreases: d electrons shield poorly. 4d ≈ 5d because of the lanthanoid contraction (Zr 160 pm, Hf 159 pm).
4 · Ionisation enthalpy. Rises, but gradually and irregularly; d electrons shield the 4s electrons.
5 · Name the exceptions.
- 2nd IE high for Cr (Cr⁺ 3d⁵) and Cu (Cu⁺ 3d¹⁰).
- 3rd IE high for Mn (Mn²⁺ 3d⁵) and Zn (Zn²⁺ 3d¹⁰); low for Fe.
6 · Link to real life. High-melting Fe, Cr and Ni make strong steel for bridges and rails.
7 · Close in one line. So stable d⁵ and d¹⁰ sets explain most of the odd values.

Scoring tip: For every trend write the reason in the same sentence; a trend without a reason earns half marks.