Define the term ionization enthalpy ? How does it vary along a period and along a group ?

Explanation

**Step-by-Step Text Solution:**

1. **Definition of Ionization Enthalpy:**
- Ionization enthalpy (or ionization energy) is defined as the minimum amount of energy required to remove an electron from the valence shell of an isolated gaseous atom. This process involves overcoming the attractive force between the negatively charged electron and the positively charged nucleus.

2. **Variation of Ionization Enthalpy Along a Period:**
- As we move from left to right across a period in the periodic table, the atomic number of the elements increases. This results in an increase in the positive charge of the nucleus (due to more protons). Consequently, the effective nuclear charge experienced by the electrons also increases.
- With an increase in effective nuclear charge, the attraction between the nucleus and the valence electrons becomes stronger, causing the atomic size to decrease. As a result, more energy is required to remove an electron, leading to an increase in ionization enthalpy across a period.
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