Which of the following statements regarding an anion is not true?

Options

A. The gain of an electron leads to the formation of an anion.
B. The radius of the anion is larger than the atomic radius of its parent atom.
C. The effective nuclear charge increases when an anion is formed
D. Electron cloud expands due to increased repulsion among the electrons.

Explanation

To determine which statement regarding an anion is not true, we will analyze each statement one by one.

### Step 1: Analyze the first statement
**Statement:** "The gain of an electron leads to the formation of an anion."
- **Analysis:** This statement is true. An anion is formed when an atom gains one or more electrons, resulting in a negative charge.

**Hint:** Remember that anions are negatively charged ions formed by the addition of electrons.

### Step 2: Analyze the second statement
**Statement:** "The radius of the anion is larger than the atomic radius of its parent ion."
- **Analysis:** This statement is also true. Anions are larger than their parent atoms because the addition of electrons increases electron-electron repulsion, causing the electron cloud to expand.

**Hint:** Compare the sizes of anions, neutral atoms, and cations to understand the trend in atomic radius.

### Step 3: Analyze the third statement
**Statement:** "The effective nuclear charge increases when anion is formed."
- **Analysis:** This statement is incorrect. When an anion is formed, the size of the electron cloud increases due to the addition of electrons, which leads to a decrease in effective nuclear charge experienced by the valence electrons. The effective nuclear charge does not increase; it actually decreases because the distance between the nucleus and the added electrons increases.

**Hint:** Recall that effective nuclear charge is influenced by both the number of protons and the distance of electrons from the nucleus.

### Step 4: Analyze the fourth statement
**Statement:** "Electron cloud expands due to increased repulsion among the electrons."
- **Analysis:** This statement is true. When an additional electron is added to form an anion, the increased repulsion among the electrons causes the electron cloud to expand.

**Hint:** Consider how adding electrons affects the forces between them.

### Conclusion
After analyzing all the statements, the statement that is not true is:
**C: "The effective nuclear charge increases when anion is formed."**

### Final Answer
The correct answer for the question is option C.