Assertion: helium atom has highest ionisation energy among all the element.
Reason: helium is smallest atom among all the elements.

Options

A. If both assertion and reason are true annd the reason is the correct explanation of assertion
B. If both assertion and reason are true but reason is not the correct explanation of assertion
C. If assertion is true but the reason is false
D. if assertion is false but the reason is true

Answer

**Step-by-Step Solution:**

1. **Understanding the Assertion:**
- The assertion states that "helium atom has the highest ionization energy among all the elements."
- Ionization energy is the energy required to remove an electron from an atom in its gaseous state.

2. **Analyzing Helium's Ionization Energy:**
- Helium has an electronic configuration of 1s², meaning it has two electrons in its outermost shell.
- This configuration is stable, and the two electrons are held tightly by the nucleus due to the effective nuclear charge.

3. **Reason for High Ionization Energy:**
- The high effective nuclear charge (the net positive charge experienced by electrons) results in a strong attraction between the nucleus and the electrons.
- Because of this strong attraction, a significant amount of energy is required to remove one of the electrons from helium, making its ionization energy the highest among all elements.

4. **Understanding the Reason:**
- The reason states that "helium is the smallest atom among all the elements."
- However, this statement is incorrect. Hydrogen, with an electronic configuration of 1s¹, is actually the smallest atom.

5. **Conclusion:**
- The assertion is true: Helium does have the highest ionization energy.
- The reason is false: Helium is not the smallest atom; hydrogen is.
- Therefore, the correct relationship is that the assertion is true, but the reason is false.

**Final Answer:**
- Assertion: True
- Reason: False
- The correct option is that the assertion is true, but the reason is false.

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