Question

What is the atomic number of the only alkali metal that does not form alum with Al2(SO4)3?

Answer 1

3

Answer 2

For alums of the general formula M(I)SO₄·M′(III)SO₄·xH₂O, the M⁺ ion must be of adequate size to stabilize the alum lattice. Lithium, having a very small ionic radius, is unable to fit properly in the lattice, and hence does not form an alum with Al₂(SO₄)₃. The atomic number of lithium is 3.