Define ionization enthalpy. Name the factors on which ionisation enthalpy depends? How does it vary down the group and across a period?

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**Step 1. Define ionization enthalpy.** The energy required to remove the most loosely-bound electron from an isolated gaseous atom in its ground state, forming a cation; always positive, expressed in kJ mol⁻¹ (section 7.5.2.3).

**Step 2. List the governing factors.** (i) Atomic/ionic radius — a larger radius means the valence electron is farther from the nucleus and easier to remove.
(ii) Effective nuclear charge $Z_{eff}$ — a higher $Z_{eff}$ holds the valence electron more tightly, raising IE.
(iii) Screening/shielding by inner (core) electrons — more shielding lowers $Z_{eff}$ and hence lowers IE.
(iv) Orbital penetration and electron-pairing — an electron in a more-penetrating orbital (e.g. 2s vs 2p) or in a singly-occupied orbital (per Hund's rule) is more tightly held, raising IE relative to a less-penetrating or paired electron.

**Step 3. State the down-group trend.** IE decreases down a group: radius increases, shielding increases, $Z_{eff}$ on the valence electron falls (Table 7.3). …

Q.Define ionization enthalpy. Name the factors on which ionisation enthalpy depends? How does it vary down the group and across a period?

Concept understanding — Ionization Enthalpy

Q.Define ionization enthalpy. Name the factors on which ionisation enthalpy depends? How does it vary down the group and across a period?

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