Does aluminum trifluoride have to have an octet around the central atom?
Answer:
\(\mathrm{AlF}_{3}\) It doesn’t have to. Neither do \(\mathrm{AlCl}_{3}, \mathrm{BF}_{3},\) etc.
Aluminum comes in with \(3\) valence electrons, and each fluorine comes in with \(7\) valence electrons. That gives
\(3+3 \times 7=\mathbf{2 4} \text { total }\) valence electrons to distribute.
Each \(F\) atom often holds three lone pairs, accounting for \((3 \times 2) \times 3=18\) of these \(24\) So, the remaining \(6\) can go into making three single bonds, each bond using \(2\) valence electrons.
And the central atom, \(Al,\) has an empty \(3 p_{z}\) orbital. That is mainly why \(\mathrm{AlF}_{3}\) is a Lewis acid.