Both alkali metals and alkaline earth metals are `s-`block elements. They resemble each other in many respects but still there are certain dissimilarities in their properties due to different number of electrons in the valence shell, different atomic radii, ionisation enthalpy, electronegativity, etc.
Like lithium, Be also differs from rest of the alkaline earth metals on account of its small atomic size and high electronetativity. `Be^(2+)`ion is very small and exerts a high polarising effect on any anion associated with it.
The correct sequence of increasing covalent character is

Options

A. `BeCI_(2) lt NaCl lt LiCl`
B. `NaCl lt LiCl lt BeCl_(2)`
C. `BeCl_(2) lt LiCl lt NaCl`
D. `LiCl lt NaCl lt BeCl_(2)`

Explanation

To determine the correct sequence of increasing covalent character among the compounds formed by alkali and alkaline earth metals with chlorine (MX), we need to analyze the factors affecting covalent character, particularly the electronegativity of the metals involved.

### Step-by-Step Solution:

1. **Understanding Covalent Character**:
The covalent character of a compound (MX) is influenced by the difference in electronegativity between the metal (M) and the non-metal (X). A smaller electronegativity difference leads to a greater covalent character.

2. **Identify the Metals and Their Electronegativity**:
- Sodium (Na) has a lower electronegativity compared to Lithium (Li) and Beryllium (Be).
- The electronegativity values are approximately:
- Na: 0.93
- Li: 1.00
- Be: 1.57

3. **Order of Electronegativity**:
Based on the electronegativity values, we can establish the order:
\[ \text{Na} < \text{Li} < \text{Be} \]
This means that Na has the least electronegativity and Be has the highest.

4. **Relating Electronegativity to Covalent Character**:
Since Na has the lowest electronegativity, the bond in NaCl will have the least covalent character. Conversely, BeCl2, with the highest electronegativity, will exhibit the most covalent character.

5. **Establishing the Order of Increasing Covalent Character**:
Therefore, the order of increasing covalent character for the compounds formed with chlorine is:
\[ \text{NaCl} < \text{LiCl} < \text{BeCl}_2 \]

6. **Final Answer**:
The correct sequence of increasing covalent character is:
\[ \text{NaCl} < \text{LiCl} < \text{BeCl}_2 \]