On reaction with dihydrogen the alkali metals

Options

A. form hydrides which are ionic solids with high melting points
B. form hydrides which are molcular solids with low melting points
C. form hydrides which are ionis solids with low melting points
D. form hydrides which are non-stoichiometric.

Explanation

**Step-by-Step Solution:**

1. **Understanding the Reaction**: The question asks about the reaction of alkali metals with dihydrogen (H₂). Alkali metals include lithium (Li), sodium (Na), potassium (K), rubidium (Rb), and cesium (Cs).

2. **Formation of Hydrides**: When alkali metals react with dihydrogen, they form hydrides. The general reaction can be represented as:
\[ 2M + H_2 \rightarrow 2MH \]
Here, M represents an alkali metal, and MH is the hydride formed.

3. **Ionic Nature of Hydrides**: The hydrides formed (MH) are ionic compounds. This means that they consist of M⁺ ions (alkali metal cations) and H⁻ ions (hydride anions).

4. **Properties of Ionic Compounds**: Ionic compounds, including the hydrides of alkali metals, typically have high melting points. This is due to the strong electrostatic forces of attraction between the positively charged cations (M⁺) and negatively charged anions (H⁻).

5. **Explanation for High Melting Points**: The high melting points can be attributed to the non-directional nature of ionic bonds. In an ionic lattice, each ion is surrounded by multiple oppositely charged ions, which stabilizes the structure and requires more energy (in the form of heat) to break these bonds.

6. **Conclusion**: Therefore, the correct statement regarding the reaction of alkali metals with dihydrogen is that they form hydrides which are ionic solids with high melting points.

**Final Answer**: Alkali metals react with dihydrogen to form hydrides (MH) which are ionic solids with high melting points.

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