In general the melting and boiling points of transition metals

Options

A. Increase gradually across the period from left to right.
B. Decrease gradually across the period from left to right.
C. First increase till the middle of the period and then increase towards the end.
D. First decrease regularly till the middle of the period and then increase towards the end.

Explanation

To answer the question regarding the variation of melting and boiling points of transition metals, we can break down the explanation into clear steps:

### Step-by-Step Solution:

1. **Understanding Transition Metals**: Transition metals are elements found in the d-block of the periodic table. They are known for their ability to form variable oxidation states and complex ions.

2. **Unpaired Electrons**: As we move across a period of transition metals, the number of unpaired electrons initially increases. This is due to the filling of the d-orbitals.

3. **Pairing of Electrons**: After reaching a certain point in the series, the unpaired electrons begin to pair up. This pairing reduces the number of unpaired electrons.

4. **Interatomic Forces**: The strength of the metallic bond, which is the force holding the metal atoms together, is influenced by the number of unpaired electrons. More unpaired electrons lead to stronger metallic bonds.

5. **Variation of Melting and Boiling Points**:
- As the number of unpaired electrons increases, the strength of the metallic bond also increases, leading to higher melting and boiling points.
- However, once the unpaired electrons start to pair up, the strength of the metallic bond decreases, resulting in lower melting and boiling points.

6. **Conclusion**: Therefore, the melting and boiling points of transition metals generally increase as we move towards the middle of the series and then decrease towards the end.

7. **Final Answer**: The correct option is that the melting and boiling points first increase till the middle of the period and then decrease towards the end.