Why is #ZnCl_2# a lewis acid?
A Lewis acid is a molecule that can accept an electron pair and a Lewis base is a molecule that can donate and electron pair. When a Lewis base combines with a Lewis acid an adduct is formed with a coordinate covalent bond.
A zinc atom has the electron configuration [Ar]4s²3d¹⁰.
Using only the s electrons, VSEPR theory predicts ZnCl₂ to have a linear AX₂ structure with only four valence shell electrons. This is an incomplete octet. Thus, ZnCl₂ will behave as a Lewis acid in an attempt to acquire more valence electrons.