Which one of the following species doesn't have a magnetic moment of 1.73 BM, (spin only value)?

Solution

Chemistry ma’am · 2 steps

1.73 BM means exactly one unpaired electron, so look for the one species that has none.

1. Decode 1.73 BM. The spin-only moment depends only on n, the number of unpaired electrons. Putting n = 1 gives √3 = 1.73 BM, so the three species that do have this moment each carry one unpaired electron.

   μ = √n(n+2) BM, n = 1 ⇒ μ = √3 = 1.73 BM

   Species	Electron count that matters	Unpaired electrons	μ (BM)
   O₂⁺	15 electrons, 1 in π^*2p	1	1.73
   CuI	Cu⁺ is 3d¹⁰	0	0
   [Cu(NH₃)₄]Cl₂	Cu²⁺ is 3d⁹	1	1.73
   O₂⁻	17 electrons, 3 in π^*2p	1	1.73

2. So. In CuI copper is Cu⁺ with a completely filled 3d¹⁰ shell, so it is diamagnetic with μ = 0. The answer is (b).

The idea to keep. For spin-only questions, first find n: 1.73 BM is one unpaired electron, 2.83 BM is two, 3.87 BM is three. For complexes, find the metal's oxidation state first, then count its d electrons.

Answer from the NTA final answer key. Solution by PrepWiser.