Concept:Ionization energy generally increases across a period. However, atoms with fully filled subshells (like \( ns^2 \)) exhibit anomalous stability, causing a spike in their ionization energy.
Formula:$$ \text{Mg: } [Ne] \ 3s^2 \quad vs \quad \text{Al: } [Ne] \ 3s^2 \ 3p^1 $$
Solution:- Sodium (Na) and Potassium (K) are Group 1 alkali metals with a single, easily lost valence electron.
- Magnesium (Mg) has a completely filled \( 3s^2 \) valence subshell, which is highly stable and strongly held by the nucleus.
- Aluminum (Al) has its outermost electron in a higher-energy \( 3p \) orbital, which is partially shielded by the \( 3s \) electrons, making it easier to remove than Mg's electron.
Why other options are incorrect:- Option A & Option D: Alkali metals have the lowest IE in their respective periods.
- Option C: Due to the \( 3p \) shielding, Al's IE drops slightly below Mg.
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