Why is the bond in beryllium chloride covalent rather than ionic?

The Be2+ cation is smaller in size (being above the rest of the group 2 elements) and hence has high charge density. This enables it to easily polarise the comparatively larger Cl- anion, which results in a covalent characteristic of the compound. Dative bonding (a special type of covalent bond in which the shared electrons come from one of the atoms only) occurs in the compound and BeCl2 usually occurs as a dimer i.e Be2Cl4 (polymeric type) based on the linear molecular structure of BeCl2.

Polarisation occurs between a small cation with high charge density and a larger anion. Since the size of the cations of the group 2 elements increases down the group (more atomic radius), charge density around the cation decreases and hence their ability to polarise the Cl- also decreases, resulting in an ionic compound.