Explain why the relative atomic mass of cobalt is greater than the relative atomic mass of nickel, even though the atomic number of nickel is greater than the atomic number of cobalt.

Deduce the numbers of protons and electrons in the ion \({\text{C}}{{\text{o}}^{2 + }}\).

Deduce the electron configuration for the ion \({\text{C}}{{\text{o}}^{2 + }}\).

cobalt has a greater proportion of heavier isotopes / OWTTE / cobalt has greater number of neutrons;

27 protons and 25 electrons;

\({\text{1}}{{\text{s}}^{\text{2}}}{\text{2}}{{\text{s}}^{\text{2}}}{\text{2}}{{\text{p}}^{\text{6}}}{\text{3}}{{\text{s}}^{\text{2}}}{\text{3}}{{\text{p}}^{\text{6}}}{\text{3}}{{\text{d}}^{\text{7}}}/{\text{[Ar] 3}}{{\text{d}}^{\text{7}}}\);

Candidates did reasonably well on this question. Many candidates got (a) correct.

Most  candidates got the correct number of protons and electrons in the \({\text{C}}{{\text{o}}^{2 + }}\) ion in part (b).

In (c), a small minority of candidates tried to answer this question with a 2,8,15 type electron arrangement which showed weakness at HL and many candidates did not realise that electrons come out of the 4s level first before the 3d in part (c).