Ionisation energy decreases in a __________ as atomic number increases. (group/period)

Explanation

To answer the question, "Ionisation energy decreases in a __________ as atomic number increases," we need to analyze the behavior of ionization energy in relation to the periodic table.

### Step-by-Step Solution:

1. **Understanding Ionization Energy**:
- Ionization energy is defined as the amount of energy required to remove an electron from the outermost shell of a gaseous atom in its ground state.

2. **Behavior in Groups**:
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