The size of an anion is larger than that of its parent atom. Give reason.

Explanation

**Step-by-Step Solution:**

1. **Definition of Anion and Parent Atom**: An anion is a negatively charged ion formed when an atom gains one or more electrons. The parent atom is the neutral atom from which the anion is derived.

2. **Example of Anion Formation**: Consider the example of oxygen (O). The neutral oxygen atom has an atomic number of 8, which means it has 8 protons and 8 electrons. When it gains two electrons to form the oxide anion (O²⁻), it now has a total of 10 electrons but still only 8 protons.

3. **Understanding Effective Nuclear Charge (Z_eff)**: The effective nuclear charge (Z_eff) is the net positive charge experienced by electrons in a multi-electron atom. It is calculated as the number of protons in the nucleus minus the shielding effect of the inner shell electrons.

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