Which element has the highest first ionisation energy?
Answer and explanation
Correct answer: A. Helium
- Step 1: First ionisation energy measures the energy required to remove one mole of electrons from one mole of gaseous atoms in the ground state.
- Step 2: Ionisation energy increases across a period and decreases down a group due to changes in effective nuclear charge and atomic radius.
- Step 3: Helium has a full valence shell, the smallest atomic radius, and maximum effective nuclear charge without shielding, giving it the highest first ionisation energy.
Answer
Helium.
Why not the others: The alternative options (Fluorine, Neon, Hydrogen) correspond to incorrect stoichiometric ratios, wrong reaction intermediates, or arithmetic evaluation errors.
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