Which of the following elements does not form hydride by direct heating with dihydrogen ?

Options

A. Be
B. Mg
C. Sr
D. Ba

Explanation

To determine which element does not form a hydride by direct heating with dihydrogen, we will analyze the elements given in the question, focusing on their ability to form hydrides based on their properties.

### Step-by-Step Solution:

1. **Identify the Elements**: The elements mentioned in the question are Beryllium (Be), Magnesium (Mg), Calcium (Ca), and Strontium (Sr). These elements belong to Group 2 of the periodic table, also known as alkaline earth metals.

2. **Understand Hydride Formation**: Alkaline earth metals typically form hydrides when heated with dihydrogen (H₂). The general formula for the hydride of alkaline earth metals is MH₂, where M is the metal.

3. **Examine Beryllium**:
- Beryllium (Be) is the first element in this group. It has a small atomic size and a high ionization enthalpy, which means it requires a significant amount of energy to remove its outermost electrons.
- Due to its small size, the attraction between the nucleus and the outermost electrons is strong, making it difficult for beryllium to lose electrons and form a hydride.

4. **Examine Other Elements**:
- Magnesium (Mg), Calcium (Ca), and Strontium (Sr) have larger atomic sizes compared to beryllium, which leads to lower ionization enthalpy.
- These elements can lose their outermost electrons more easily and thus can form hydrides (MgH₂, CaH₂, SrH₂) when heated with dihydrogen.

5. **Conclusion**: Based on the analysis, Beryllium (Be) does not form a hydride when heated with dihydrogen due to its high ionization enthalpy and small atomic size. Therefore, the correct answer is Beryllium.

### Final Answer:
**Beryllium (Be) does not form hydride by direct heating with dihydrogen.**

---