Establish the type of compound that BeCl2 is and the informal charges on it.
Beryllium is an alkaline Earth metal found in group 2 on the periodic table. It readily loses its two valence electrons to become the Be2+ cation. Chlorine is a group 17 halogen on the periodic table. With seven valence electrons, it readily accepts an additional electron to become the Cl- anion. All of this means that beryllium chloride (BeCl2) is an inorganic compound composed of one beryllium atom and two chlorine atoms. Based on the above valency information, one might assume that it is a salt composed of the ions described above. But the beryllium cation is so small that it actually tends to form more covalent type structures. So a molecule of BeCl2 consists of a central Be atom flanked on either side by two Cl atoms that it makes mostly covalent bonds with. The shape of the molecule is linear as predicted by VSEPR theory. In terms of informal charges, you can think of the Be as having a 2+ charge and the chlorines as having a 1- charge each to sum to a total of 0 (neutral compound), but in reality the two Be valence electrons make more of a covalent bond with the chlorines. This does leave the central Be atom as electron deficient (it does not have an octet of electrons surrounding it) Lewis acid, so as a solid it tends to align itself in polymeric chains whereby lone pairs of electrons from neighboring chlorine atoms coordinate with it to help satisfy the electron deficiency.
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