Question

Phosphorus has three electrons in its 3p sublevel and sulfur has four.

Phosphorus should have the lower ionization energy but it does not,
sulfur does. What does this provide evidence for?
A. Aufbau's principle
B. Conservation of energy
C. Hund' rule
D. None of these
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Answer 1

The correct answer is C. Hund's rule.

Hund's rule states that for atoms in their ground state, electrons will occupy orbitals of the same energy level one by one before pairing up. This means that when filling the 3p sublevel, phosphorus should have one electron in each of the three 3p orbitals before any of them start pairing up.

However, in reality, sulfur has a lower ionization energy than phosphorus. This means that it is easier to remove an electron from sulfur than from phosphorus. This contradicts the expectation based on Hund's rule.

Therefore, the fact that sulfur has a lower ionization energy than phosphorus provides evidence that Hund's rule is not the only factor governing the electron configuration and ionization energies of elements. Other factors, such as electron-electron repulsion and the stability of half-filled or filled subshells, also play a role.