Beryllium shows different properties from rest of the alkaline earth metals because of its small atomic and ionic size, high
electronegativity and high polarising power.But beryllium shows similarities in properties to the second member of the next
group i.e. aluminium on account of similar atomic and ionic size and nearly same polarising power
Which of the following statements is true ?

Options

A. BeO and `Be(OH)_2` both are purely basic in nature
B. Be can expand its covalency to six
C. Be largely forms covalent compounds which get easily hydrolysed
D. Be combines with hydrogen upon heating to form `BeH_2`

Explanation

To solve the question regarding the properties of Beryllium and its comparison with other alkaline earth metals and Aluminium, we need to analyze the statements provided based on the information given in the prompt.

### Step-by-Step Solution:

1. **Understanding Beryllium's Unique Properties**:
- Beryllium (Be) is different from other alkaline earth metals due to its small atomic and ionic size, high electronegativity, and high polarizing power.
- These characteristics lead to Beryllium exhibiting different chemical behavior compared to its group members (Magnesium, Calcium, etc.).

**Hint**: Remember that Beryllium's unique properties stem from its position in the periodic table and its atomic structure.

2. **Diagonal Relationship with Aluminium**:
- Beryllium shows similarities to Aluminium (Al) because of their similar atomic and ionic sizes and nearly the same polarizing power.
- This diagonal relationship is significant in predicting the chemical behavior of these elements.

**Hint**: Diagonal relationships in the periodic table often lead to similarities in properties between elements in different groups.

3. **Analyzing the Statements**:
- **Statement A**: "BEO and BEOH2 both are purely basic in nature."
- This is **incorrect**. Beryllium oxide (BeO) and Beryllium hydroxide (Be(OH)2) are amphoteric, meaning they can react with both acids and bases.

**Hint**: Amphoteric substances can act as both acids and bases, which is crucial for understanding their behavior.

- **Statement B**: "B can expand its valency, covalency to 6."
- This is **incorrect**. Beryllium cannot expand its covalency beyond 4 because it lacks d-orbitals in its valence shell.

**Hint**: The ability to expand covalency is often linked to the presence of d-orbitals in the valence shell.

- **Statement C**: "Beryllium forms covalent compounds which get easily hydrolyzed."
- This is **correct**. Beryllium compounds are covalent and have a high charge density, leading to easier hydrolysis.

**Hint**: Covalent character in compounds often leads to different reactivity compared to ionic compounds.

- **Statement D**: "B combines with hydrogen upon heating to form BH2."
- This is **incorrect**. Beryllium does not readily combine with hydrogen to form hydrides like other alkaline earth metals do.

**Hint**: Check the reactivity of elements with hydrogen to determine their ability to form hydrides.

4. **Conclusion**:
- The only true statement among the options provided is **Statement C**: "Beryllium forms covalent compounds which get easily hydrolyzed."

### Final Answer:
The true statement is that Beryllium forms covalent compounds which get easily hydrolyzed.