`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 magnesium (Mg), barium (Ba), beryllium (Be), and calcium (Ca) cannot undergo the reaction with water at high temperatures, we can follow these steps:

### Step-by-Step Solution:

1. **Understand the Reaction**:
The general reaction is:
\[ M + 2H_2O \rightarrow M(OH)_2 + H_2 \]
This indicates that a metal (M) reacts with water to form a metal hydroxide and hydrogen gas.

2. **Identify the Metals**:
The metals in question are:
- Magnesium (Mg)
- Barium (Ba)
- Beryllium (Be)
- Calcium (Ca)

3. **Consider Ionization Energy**:
The ability of a metal to react with water is related to its ionization energy. Metals with lower ionization energy can lose electrons more easily and thus can react with water.

4. **Order the Metals by Atomic Number**:
In the periodic table, the order of these metals based on their atomic number is:
- Beryllium (Be)
- Magnesium (Mg)
- Calcium (Ca)
- Barium (Ba)

5. **Analyze the Size and Ionization Energy**:
- As we move down the group in the periodic table, the atomic size increases, and thus the ionization energy decreases.
- Beryllium has a smaller atomic size and higher ionization energy compared to the other metals listed.

6. **Conclusion**:
Since beryllium has a higher ionization energy, it is less likely to lose electrons and thus cannot undergo the reaction with water at high temperatures. Therefore, the metal that cannot undergo this reaction is:
\[ \text{Beryllium (Be)} \]

### Final Answer:
**Beryllium (Be)** cannot undergo the reaction at high temperatures.