IB DP Chemistry - Structure 3.1 The periodic table: Classification of elements- IB Style Questions For HL Paper 1A -FA 2025
▶️ Answer/Explanation
Reactivity depends on:
- Metal: K is more reactive than Na (lower ionization energy)
- Halogen: Cl2 is more reactive than Br2 (higher electronegativity)
Most vigorous reaction: Most reactive metal (K) + most reactive halogen (Cl2)
✅ Answer: (B)
▶️ Answer/Explanation
The first ionization energy is the energy needed to remove one electron from a gaseous atom. Across a period, it generally increases because the nuclear charge increases and electrons are held more strongly.
Phosphorus has the electron configuration ending in 3p³, with three unpaired electrons in the outer p sub-level (one in each p orbital). This half-filled arrangement is relatively stable and experiences less electron–electron repulsion.
Sulfur has the configuration ending in 3p⁴, meaning one of the p orbitals now contains a pair of electrons. The paired electrons in the same orbital repel each other, so it takes less energy to remove one of them. This extra repulsion lowers the first ionization energy of sulfur compared with phosphorus.
Statement (B) correctly captures this idea: phosphorus does not have paired electrons in the outer p sub-level, whereas sulfur does, leading to a lower first ionization energy for sulfur.
✅ Answer: (B)
