Question

Which of the alkaline earth metal halides given below is essentially covalent in nature?
A) $BeC{l_2}$
B) $MgC{l_2}$
C) $SrC{l_2}$
D) $CaC{l_2}$

Answer 1

Hint: You must know that according to Fajan’s rule, a small positive ion and a large negative ion favours covalency. High charge on cation increases its polarising power and high charge on anion increases its polarisability. When polarisation increases, covalency or covalent character is favoured.

Complete step by step solution:
Group 2 elements are called alkaline earth metals and consist of beryllium (Be), magnesium (Mg), calcium (Ca), strontium (Sr), and barium (Ba). Alkaline earth metals combine with halogen at elevated temperature to form halides.
$M + {X_2} \to M{X_2}$
Where, M is any alkaline earth metal and X is the halogen.
In alkaline earth metal halides, metal behaves as cation with +2 charge and halogen behave as an anion with -1 charge. Now, when positive ion (cation) is strong enough to polarise negative ion (anion), it will be able to pull electron density away from negative ion, as a result of which covalent character is introduced in their ionic bond. Fajan’s rules which summarise the factors favouring polarisation and covalency are:
- Smaller the size of the cation and the larger the size of the anion, greater is the covalent character of an ionic bond.
- Higher the charge on the cation, the greater is the covalent character.
We all know that as we move down the group, the size of the elements increases. Among all the alkaline earth metal halides, in $BeC{l_2}$ molecule, $B{e^{2 + }}$ ion is the smallest in size and also has the higher charger on it. Hence, it will be able to pull the electron density away from the anion which is $C{l^{ - 1}}$ and thus introduce the covalent character. That’s why $BeC{l_2}$ is covalent in nature.

Thus, option A is correct.

Note: When a positive ion (cation) comes closer to the negative charge (anion), it attracts electron density of the negative ion and negative ion attracts the nucleus of positive ion. Thus, each of them distorts the harmony of the other. This disturbance is called polarisation and the ability to get polarised is called polarisability. The polarising power of the cation, the polarisability of the anion, and the extent of distortion i.e., polarisation of anion are the factors which determine the percent covalent character of an ionic bond.