Assertion : The atomic radii of elements decrease on moving from left to
Reason : On moving from left to right in a period, the ionisation enthalpies right in a period. increase with increase in atomic number.

Answer

To solve the question, we need to analyze both the assertion and the reason provided.

### Step 1: Understand the Assertion
The assertion states that "the atomic radii of elements decrease on moving from left to right in a period."

- **Explanation**: As we move from left to right in a period, the number of protons in the nucleus increases, which leads to an increase in the nuclear charge. At the same time, electrons are added to the same energy level (shell). The increased nuclear charge pulls the electrons closer to the nucleus, resulting in a decrease in atomic radius.

### Step 2: Understand the Reason
The reason states that "on moving from left to right in a period, the ionization enthalpies increase with an increase in atomic number."

- **Explanation**: Ionization enthalpy is the energy required to remove an electron from an atom. As the atomic radius decreases (due to increased nuclear charge), the electrons are held more tightly by the nucleus. This makes it more difficult to remove an electron, thus increasing the ionization enthalpy.

### Step 3: Analyze the Relationship
Now, we need to determine if the reason correctly explains the assertion.

- While both the assertion and reason are true, the reason provided does not directly explain why atomic radii decrease. The decrease in atomic radius is primarily due to the increased effective nuclear charge, while the increase in ionization enthalpy is a consequence of the decreased atomic radius.

### Conclusion
Both the assertion and reason are correct, but the reason is not the correct explanation for the assertion. Therefore, the answer to the question is that both the assertion and reason are true, but the reason does not correctly explain the assertion.

### Final Answer
Both assertion and reason are true, but the reason is not the correct explanation of the assertion.

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