Configurations, oxidation states and who counts as a transition element
Answers are for parents and teachers.
1
Write the configuration of V³⁺ and count its unpaired electrons. (Atom of vanadium: [Ar] 3d³ 4s².)
Remove both 4s electrons and one 3d electron: [Ar] 3d². Two electrons in separate boxes, so 2 unpaired.
2
Which is easier to oxidise, Fe²⁺ to Fe³⁺ or Ni²⁺ to Ni³⁺? Give the d configurations as evidence.
Fe²⁺ (3d⁶) → Fe³⁺ (3d⁵) gives a half-filled sublevel, so it is easier. Ni²⁺ (3d⁸) → Ni³⁺ (3d⁷) gives no special arrangement.
3
Find the oxidation state of chromium in Cr₂O₃ and of manganese in MnO₂.
Cr₂O₃: 2x + 3 × (−2) = 0, so x = +3. MnO₂: x + 2 × (−2) = 0, so x = +4.
4
Decide which of Cd, Ni, Sc, Zn and Au are transition elements and give the reason in a few words for each.
Ni: atom 3d⁸, yes. Sc: atom 3d¹, yes. Au: Au³⁺ is 5d⁸, yes. Cd and Zn: atom and ion both d¹⁰, no.
5
A pupil writes chromium as [Ar] 3d⁴ 4s². Correct this in two sentences.
The real configuration is [Ar] 3d⁵ 4s¹. One 4s electron moves into 3d to make a half-filled 3d sublevel, which is a little more stable.