Sketch a graph for the variation of first ionization energy against atomic number for the elements across period 3 of the periodic table (Na - Ar) and explain any anomalies.

SECTION B: INORGANIC CHEMISTRY

3. (a) This question concerns the elements across period 3 of the periodic table (Na - Ar).

(i) Sketch a graph for the variation of first ionization energy against atomic number for the elements.

• The graph generally shows an increasing trend across the period due to increasing nuclear charge and decreasing atomic radius.
• There are drops at Aluminium (Al) and Sulfur (S) due to electron configuration effects.
  • The drop from Magnesium (Mg) to Aluminium (Al) is because Al has its outermost electron in a $3\text{p}$ orbital, which is higher in energy and shielded by the $3\text{s}^2$ electrons, making it easier to remove than a $3\text{s}$ electron from Mg.
  • The drop from Phosphorus (P) to Sulfur (S) is because S has a paired electron in one of its $3\text{p}$ orbitals. Removing this paired electron (which experiences repulsion) is easier than removing an unpaired electron from the half-filled $3\text{p}$ subshell of P.

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