`M + 2H_(2)O rarr M(OH)_(2) + H_(2)`
Which metal among the following cannot undergo this reacton at high temperatures ?

Options

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

Answer

To determine which metal among the given options cannot undergo the reaction with water at high temperatures, we need to analyze the properties of the metals involved, particularly their ionization energies. The reaction in question is:

\[ M + 2H_2O \rightarrow M(OH)_2 + H_2 \]

### Step-by-Step Solution:

1. **Identify the Metals**: The metals mentioned are typically from Group 2 of the periodic table, which includes Beryllium (Be), Magnesium (Mg), Calcium (Ca), and Barium (Ba).

2. **Understand the Reaction**: In this reaction, the metal (M) reacts with water to form metal hydroxide and hydrogen gas. For this reaction to occur, the metal must lose two electrons to form a \( M^{2+} \) ion.

3. **Ionization Energy Concept**: The ability of a metal to react with water is largely dependent on its ionization energy. Metals with lower ionization energies can lose electrons more easily and will therefore react more readily with water.

4. **Compare Ionization Energies**:
- **Beryllium (Be)**: Has the highest ionization energy among the group due to its smaller atomic size and strong nuclear attraction.
- **Magnesium (Mg)**: Has a lower ionization energy than Beryllium but higher than Calcium and Barium.
- **Calcium (Ca)**: Has a lower ionization energy than Magnesium.
- **Barium (Ba)**: Has the lowest ionization energy among these metals.

5. **Conclusion**: Since Beryllium has the highest ionization energy, it is the least likely to lose electrons and thus cannot undergo the reaction with water at high temperatures. Therefore, it does not react to form \( Be(OH)_2 \) and hydrogen gas.

### Final Answer:
The metal that cannot undergo this reaction at high temperatures is **Beryllium (Be)**.