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Among the atoms Be, B, C, N, O, F, Cl, S: identify the atom with the highest electron affinity and the atom with the lowest ionisation energy. What is the shape of the binary molecule formed between these two atoms?

1. Linear
2. Tetrahedral
3. Trigonal pyramidal
4. Trigonal planar

Correct answer: Trigonal planar

Solution

Among {Be, B, C, N, O, F, Cl, S}, Cl has the greatest electron affinity (~349 kJ/mol, higher than F due to smaller electron-electron repulsion in the compact F 2p orbital). B has the lowest first ionisation energy (~800 kJ/mol). The binary molecule formed by B and Cl is BCl3. Boron uses sp² hybridisation, has 3 bond pairs and 0 lone pairs around the central atom, giving a trigonal planar shape with 120 deg bond angles.

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