Which of the following statements regarding the variation of atomic radii in the periodic table is not true?

Options

A. In a group, there is continuous decrease in size with increase in atomic number.
B. The size of inert gases is larger than halogens.
C. The size of inert gases is larger than halogens.
D. In `3^(rd)` period, the size of atoms increase with increase in atomic number.

Explanation

To determine which statement regarding the variation of atomic radii in the periodic table is not true, we will analyze each option provided.

### Step-by-Step Solution:

1. **Understanding Atomic Radii Trends**:
- Atomic radii generally decrease across a period from left to right due to the increasing nuclear charge which pulls the electrons closer to the nucleus.
- Atomic radii increase down a group because additional electron shells are added, which outweighs the increased nuclear charge.

2. **Analyzing Each Statement**:
- **Option 1**: "In a group, there is a continuous decrease in size with an increase in atomic number."
- This statement is **not true**. In reality, atomic size increases down a group as additional electron shells are added.

- **Option 2**: "The size of inert gases is larger than halogens."
- This statement is **true**. Inert gases have completely filled electron shells, leading to a larger atomic radius compared to halogens, which have partially filled shells.

- **Option 3**: "In the third period, the size of the atom increases with an increase in atomic number."
- This statement is **not true**. In the third period, atomic size actually decreases as you move from sodium (Na) to argon (Ar) due to increasing nuclear charge.

- **Option 4**: "In the third period, the size of the atom decreases with an increase in atomic number."
- This statement is **true**. As explained, atomic size decreases across a period.

3. **Conclusion**:
- The statements that are not true are Options 1 and 3. However, since the question asks for a single statement that is not true, we can conclude that **Option 1** is the most straightforwardly incorrect regarding the trend in atomic radii in groups.

### Final Answer:
The statement that is not true is: **Option 1: In a group, there is a continuous decrease in size with an increase in atomic number.**

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