Number of molecules/species from the following having one unpaired electron is ________.
O₂, O₂⁻¹, NO, CN⁻¹, O₂²⁻

Solution

Chemistry ma’am · 3 steps

Count total electrons, then see how many sit in the two π^* orbitals. That decides everything here.

1. Count electrons. Add the electrons of both atoms, then adjust for charge. Each negative charge adds one electron.
2. Find the frontier. For 14 electrons, every bonding orbital up to π 2p is full. Electrons 15 to 18 go into the two degenerate π^* 2p orbitals, one in each first (Hund's rule) and then pairing up.

   Species	Total electrons	Electrons in π^* 2p	Unpaired
   O₂	8 + 8 = 16	↑ ↑	2
   O₂⁻	16 + 1 = 17	↑↓ ↑	1
   NO	7 + 8 = 15	↑	1
   CN⁻	6 + 7 + 1 = 14	empty	0
   O₂²⁻	16 + 2 = 18	↑↓ ↑↓	0

3. So. Only O₂⁻ and NO have exactly one unpaired electron, so the answer is 2.

The common slip: 3. Counting every paramagnetic species, including O₂, gives 3. But O₂ has two unpaired electrons, not one.

The idea to keep. For diatomics of second-period atoms: 14 electrons means none unpaired, 15 or 17 means one, 16 means two, 18 means none. Just count the electrons in π^*.

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