Does aluminum trifluoride have to have an octet around the central atom?
#"AlF"_3#? It doesn't have to. Neither do #"AlCl"_3#, #"BF"_3#, Etc.
Aluminum comes in with #bb3# elettroni di valenza, and each fluorine comes in with #bb7# elettroni di valenza. That gives
#3 + 3 xx 7 = bb24# total valence electrons to distribute.
Ogni #"F"# atom often holds three lone pairs, accounting for #(3 xx 2) xx 3 = 18# di questi #24#. So, the remaining #6# can go into making three single bonds, each bond using #2# elettroni di valenza.
And the central atom, #"Al"#, has an empty #3p_z# orbital. That is mainly why #"AlF"_3# รจ un acido di Lewis.