With reference to the variation of properties in the periodic table, which of the following is true?

Options

A. Atomic size decreases from left to right across a period.
B. Ionisation energy decreases from left to right across a period.
C. Electron affinity increases on going down a group.
D. Electronegativity decreases from left to right across a period.

Explanation

To determine which of the statements regarding the variation of properties in the periodic table is true, we will analyze each option step by step.

### Step-by-Step Solution:

1. **Option 1: Atomic size decreases from left to right across a period.**
- **Analysis:** This statement is true. As we move from left to right across a period, the number of protons in the nucleus increases, which leads to a stronger positive charge. This increased nuclear charge pulls the electrons closer to the nucleus, resulting in a decrease in atomic size.
- **Conclusion:** True.

2. **Option 2: Ionization energy decreases from left to right across a period.**
- **Analysis:** This statement is false. Ionization energy is the energy required to remove an electron from an atom. As we move from left to right across a period, the nuclear charge increases, making it harder to remove an electron. Therefore, ionization energy actually increases from left to right.
- **Conclusion:** False.

3. **Option 3: Electron affinity increases on going down a group.**
- **Analysis:** This statement is also false. Electron affinity is the energy change when an electron is added to a neutral atom. Generally, electron affinity decreases as you move down a group because the added electrons are further from the nucleus and experience more shielding from inner electrons, making them less attracted to the nucleus.
- **Conclusion:** False.

4. **Option 4: Electronegativity decreases from left to right across a period.**
- **Analysis:** This statement is false. Electronegativity is a measure of an atom's ability to attract electrons in a bond. As we move from left to right across a period, electronegativity increases due to the increasing nuclear charge, which enhances the atom's ability to attract electrons.
- **Conclusion:** False.

### Final Conclusion:
The only true statement among the options is **Option 1: Atomic size decreases from left to right across a period.**

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