Melting and boiling points increase regularly in going across the second period. Explain.

Answer

### Step-by-Step Solution:

1. **Identify the Elements in the Second Period:**
The second period of the periodic table includes the following elements: Lithium (Li), Beryllium (Be), Boron (B), Carbon (C), Nitrogen (N), Oxygen (O), Fluorine (F), and Neon (Ne).

2. **Observe the Trend in Melting and Boiling Points:**
As we move from Lithium to Neon, the melting and boiling points of these elements show a regular increase. Lithium has the lowest melting and boiling points, while Neon has the highest.

3. **Understanding the Initial Increase:**
The first few elements in the second period (Li and Be) are metals. The metallic bond strength increases from Lithium to Beryllium, which contributes to higher melting and boiling points. Metals generally have higher melting and boiling points due to stronger metallic bonds.

4. **Transition to Non-Metals:**
As we move from Beryllium to Boron and then to Carbon, we transition from metallic to non-metallic character. The nature of bonding changes from metallic to covalent.

5. **Increase in Electrons:**
As we move from left to right across the period, each element gains an additional electron. This increase in the number of electrons leads to greater delocalization of electrons, especially in non-metals.

6. **Delocalization and Bond Strength:**
The increase in delocalized electrons enhances the bond strength between atoms. Stronger bonds result in higher melting and boiling points.

7. **Conclusion:**
Therefore, the regular increase in melting and boiling points across the second period can be attributed to the increasing strength of metallic bonds in metals and the increasing bond strength due to delocalized electrons in non-metals.

### Summary:
The melting and boiling points increase across the second period due to:
- Increased metallic bond strength in metals (Li to Be).
- Transition to covalent bonding with increased electron delocalization in non-metals (B to Ne).

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