2022 H2 Chem P1 Q30 — Transition elements Solution

The answer is D.

Fe is [Ar]\(3d^64s^2\) and Co is [Ar]\(3d^74s^2\). The first ionisation energy removes a 4s electron, since the 4s electrons are the outermost.
Co has one more proton than Fe, but it also has one more 3d electron. The 3d electrons are in the penultimate shell and shield the outer 4s electrons from the nucleus, so the extra 3d electron largely offsets the extra nuclear charge. The effective nuclear charge felt by the 4s electron is similar, so the first ionisation energies (and the atomic radii) are similar.

• A is wrong: the two 4s electrons are in the same shell and provide the same small amount of shielding in both atoms, which cannot offset Co's extra nuclear charge; the similarity comes from the extra inner 3d electron shielding the 4s electrons in Co.
• B is wrong: Co has no 4p electrons; its extra electron is in the 3d subshell.
• C is wrong: the electron removed first is a 4s electron, not a 3d electron, because the 4s electrons are the outermost.