Generally, the first ionization energy increases along a period. But there are some exceptions. One which is NOT an exception is……………

Options

A. N and O
B. Na and Mg
C. Mg and Al
D. Be and B

Explanation

To solve the question about the exceptions in the trend of first ionization energy across a period, we can follow these steps:

### Step 1: Understand the Trend
The first ionization energy generally increases as we move from left to right across a period in the periodic table. This is due to the increase in effective nuclear charge, which makes it harder to remove an electron.

**Hint:** Remember that effective nuclear charge increases across a period, leading to higher ionization energies.

### Step 2: Identify the Elements
We need to identify the elements mentioned in the question. The key elements to consider are:
- Lithium (Li)
- Sodium (Na)
- Beryllium (Be)
- Magnesium (Mg)
- Boron (B)
- Aluminum (Al)
- Carbon (C)
- Nitrogen (N)
- Oxygen (O)

**Hint:** Write down the elements in order of their atomic number to visualize their positions in the periodic table.

### Step 3: Analyze the Exceptions
We need to look for exceptions in the trend of ionization energy:
- **Nitrogen (N) vs. Oxygen (O):** Oxygen has a lower ionization energy than nitrogen due to its half-filled p subshell (which is more stable).
- **Magnesium (Mg) vs. Sodium (Na):** Magnesium has a higher ionization energy than sodium, which follows the trend.
- **Beryllium (Be) vs. Boron (B):** Beryllium has a higher ionization energy than boron due to its fully filled subshell.

**Hint:** Compare the ionization energies of adjacent elements to identify exceptions.

### Step 4: Determine Which is NOT an Exception
From the analysis:
- Sodium (Na) does not exhibit an exception in the trend compared to magnesium (Mg).
- Therefore, the correct answer to the question is Sodium (Na).

**Hint:** Look for pairs of elements where the trend is maintained without any exceptions.

### Final Answer
The element that is NOT an exception to the trend of increasing first ionization energy across a period is **Sodium (Na)**.