Which of the follwing forms covalent compound?

Options

A. `Be`
B. `Mg`
C. `Ca`
D. `Sr`

Explanation

To determine which of the following alkaline earth metals forms covalent compounds, we need to analyze the properties of each element: beryllium, magnesium, calcium, and strontium.

### Step-by-Step Solution:

1. **Identify the Elements**: The elements in question are beryllium (Be), magnesium (Mg), calcium (Ca), and strontium (Sr). All of these elements belong to Group 2 of the periodic table, known as alkaline earth metals.

2. **Understand Atomic Size Trends**: As we move down the group from beryllium to strontium, the atomic size increases. This is due to the addition of electron shells, which leads to a decrease in the effective nuclear charge experienced by the outermost electrons.

3. **Consider Polarizing Power**: The smaller the cation, the higher its polarizing power. Polarizing power refers to the ability of a cation to distort the electron cloud of an anion. Beryllium, being the smallest cation among the group, has a high polarizing power.

4. **Covalent Character**: According to Fajan's rules, smaller cations with high polarizing power tend to form covalent bonds because they can effectively attract and share electrons with anions. Beryllium, with its small size and high polarizing power, can easily share its two valence electrons, leading to the formation of covalent compounds.

5. **Comparison with Other Elements**: In contrast, magnesium, calcium, and strontium have larger atomic sizes and lower polarizing power. As a result, they are more likely to form ionic compounds rather than covalent ones.

6. **Conclusion**: Based on the analysis, beryllium is the only element among the options that forms covalent compounds due to its small size and high polarizing power.

### Final Answer:
Beryllium (Be) forms covalent compounds.