Draw the Lewis structure of `BeCl_2`.

Explanation

Step 1: The number of valence electrons is `16:(2xx7)=14` from two Cl atoms and 2 from `Be` (which is in group IIA).
Step 2: Since Cl is more electronegative than `Be`, chlorine cannot form more than one covalent bond with `Be`. Thus, the only possible skeleton structure is
`Cl-Be-Cl`
Note that Cl forms more than one covalent bond only when it interacts with highly electronegative elements like O or F which help in the excitation of its valence electrons.
Step 3: Since two single covalent bonds are involved in the skeleton structure, `(16-4=)12` electrons are left to be accounted for
Step 4: Since each Cl forms one bond and Be forms two bonds, a total of `6+4+6=16` electrons is needed to complete all octets. Only 12 valence electrons are available.
Step 5: Since the number of remaining electrons is four less than the number of electrons needed to complete all octets, two more bonds are required, i.e., two extra bonds would be added because of the shortage of four electrons. Since Cl can form only one bond, this solution is not possible. Thus, `BeCl_2` molecule must have an incomplete octet.
The Be atom, which is less electronegative than Cl, has incomplete octet, giving the Lewis structure:
`:overset(..)underset(..)Cl-Be-overset(..)underset(..)Cl:`
Beryllium is a group IIA (Gr. 2) and a second period element. The electron configuration of `Be` is `1s^2 2s^2`. Thus, it has only two valence electrons in the `2s` orbital. In the gas phase, beryllium hydride `(BeH_2)` exists as discrete molecules. The Lewis structure of `BeH_2` is
`H-Be-H`
As we can see, only four valence electrons surrounded the Be atom, and there is no way to satisfy the octet rule for Be in this molecule. We might expect a similar situation for the compounds of the other group IIA elements: `Mg, Ca, Sr, Ba`, and `Ra`. However, these metallic elements have lower ionization enthalpies on account of larger radii, so they usually form ions (with complete octet) by losing two electrons, e.g., in `MgCl_2`, the `Mg^(2+)` ions has 8 valence electrons (octet).
Lithium metal `(1s^2 2s^1)` of group IA (alkali metals) forms mainly ionic compounds by losing its valence electron. The `Li^(+)` ion so formed has He configuration and has just two valence electrons in its valence shell. Thus, in lithium chloride (LICI), chlorine has complete octet while Li has incomplete octet.
Elements in group IIIA (Gr. 13), particularly B and Al, also tend to form compounds such as `BCl_3`, `AlCl_3`, etc., in which less than eight electrons surround the central atom.
Compounds such as `BeCl_2`, `BCl_3`, and `AlCl_3` in which the central atom shares less than 8 electrons are sometimes referred to as electron-deficient compounds. This deficiency refers only to satisfying the octet rule for the central atom. The term does not imply that there are less electrons than there are protons in the nuclei (as in the case of a cation) because the molecule is neutral.