Question

Boron forms BF₄^- ion while aluminium forms AlF_{6^{3- ion,though they are in same group.why?}}

Answer 1

Boron is unable to expand its octet due to non-availability of d -orbitals. The maximum covalence of boron cannot exceed 4.
principal quantum number (n) =No.of shells
l( azimuthal quantum number (refers to subshell) value ranges from 0..n-1
Boron does not have d-orbital (1s2 2s2 2p1) having n=2,cannot have l=2 so can not form sub octet species ;Hence estricted to form BF₄^- ,whereas,aluminium on other hand (1s2 2s2 2p6 3s2 3p1) n=3,can have l=2.Hence there is presence of empty d-orbitals and can extend its covalency =4 and form sub octet species such as [AlF_{6^{3- ].}}