S3.1.7 Discontinuities in Ionization Energy Trends (Higher Level Only)

Explanation of Discontinuities in Ionization Energy Trends

General Trend Across a Period

1. As you move across a period in the periodic table:
   1. Nuclear charge increases (more protons in the nucleus).
   2. Electrons are added to the same principal energy level, so shielding remains relatively constant.
   3. The increased attraction between the nucleus and the outermost electrons results in a higher ionization energy.
2. However, this smooth increase is interrupted at specific points due to the stability of half-filled and fully filled sublevels.

Stability of Half-Filled and Fully Filled Sublevels

1. Electrons in an atom are arranged in sublevels (s, p, d, etc.), which have distinct energy levels.
2. The stability of an atom's electron configuration depends on how these sublevels are filled.
3. Two configurations are particularly stable:
   1. Half-filled sublevels: Sublevels where each orbital contains one electron (e.g., p³ or d⁵).
   2. Fully filled sublevels: Sublevels where all orbitals are completely filled (e.g., p⁶ or d¹⁰).
4. This stability arises from:
   1. Symmetry: Half-filled and fully filled sublevels have symmetrical electron distributions, which lower the energy of the atom.
   2. Exchange energy: Electrons in half-filled sublevels can exchange positions within orbitals, which increases stability due to reduced electron repulsion.

Key Discontinuities in Ionization Energy

1. Between Group 2 and Group 13 Elements

1. Example: $Be$ (Group 2) vs. $B$ (Group 13)
2. Observation: The ionization energy of $B$ is lower than expected, despite the general trend of increasing ionization energy across a period.
3. Explanation:
   1. $Be$ has a fully filled 2s sublevel (2s²), which is particularly stable.
   2. $B$ has an electron in the 2p sublevel (2s² 2p¹).
   3. The 2p electron is higher in energy and experiences less nuclear attraction due to shielding by the 2s electrons.
   4. This makes it easier to remove, resulting in a lower ionization energy.

2. Between Group 15 and Group 16 Elements

1. Example: $N$ (Group 15) vs. $O$ (Group 16)
2. Observation: The ionization energy of $O$ is lower than expected.
3. Explanation:
   1. $N$ has a half-filled 2p sublevel (2p³), which is particularly stable.
   2. $O$ has one more electron (2p⁴), which introduces electron-electron repulsion within the 2p orbital.
   3. This repulsion makes it easier to remove an electron from $O$, resulting in a lower ionization energy.

Evidence for Sublevels: Ionization Energy Trends

1. The periodic trends in ionization energy provide strong evidence for the existence of sublevels (s, p, and d).
2. If electrons were arranged in a simple, uniform manner, ionization energy would increase smoothly across a period.
3. However, the observed discontinuities reveal the presence of distinct sublevels with varying stability.

How Ionization Energy Trends Support Sublevels

1. Sharp Increases Between Energy Levels
   1. A significant jump in ionization energy occurs when removing an electron from a lower principal energy level (e.g., from 2p to 1s).
   2. This indicates that electrons are organized into distinct energy levels and sublevels.
2. Discontinuities Within a Period
   1. The deviations in ionization energy trends (e.g., between Groups 2 and 13 or Groups 15 and 16) align with the filling of sublevels (s, p, d).
   2. This supports the idea that sublevels have different energy levels and stability.
3. Transition Metals and d Sublevels
   1. The relatively small changes in ionization energy across the transition metals (d-block) reflect the filling of the d sublevel, which is closer in energy to the s sublevel.
   2. This provides further evidence for the existence of d sublevels.