Define ionisation energy. What is the value for a hydrogen atom?

Explanation

**Step-by-Step Solution:**

1. **Definition of Ionization Energy:**
- Ionization energy is defined as the minimum amount of energy required to remove an electron from an atom or ion in its gaseous state. This energy is necessary to overcome the attractive forces between the negatively charged electron and the positively charged nucleus.

2. **Understanding the Concept:**
- When an electron is removed from an atom, it is said to be ionized. The energy required to achieve this is the ionization energy. It is measured in electron volts (eV) or joules (J).

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