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

Explanation of Discontinuities in Ionization Energy Trends

General Trend Across a Period

As you move across a period in the periodic table:

• Nuclear charge increases (more protons in the nucleus).
• Electrons are added to the same principal energy level, so shielding remains relatively constant.
• The increased attraction between the nucleus and the outermost electrons results in a higher ionization energy.

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

• Electrons in an atom are arranged in sublevels (s, p, d, etc.), which have distinct energy levels.
• The stability of an atom's electron configuration depends on how these sublevels are filled. Two configurations are particularly stable:
  • Half-filled sublevels: Sublevels where each orbital contains one electron (e.g., p³ or d⁵).
  • Fully filled sublevels: Sublevels where all orbitals are completely filled (e.g., p⁶ or d¹⁰).

This stability arises from:

• Symmetry: Half-filled and fully filled sublevels have symmetrical electron distributions, which lower the energy of the atom.
• 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

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

2. Between Group 15 and Group 16 Elements

• Example: N (Group 15) vs. O (Group 16)
• Observation: The ionization energy of O is lower than expected.
• Explanation:
  • N has a half-filled 2p sublevel (2p³), which is particularly stable.
  • O has one more electron (2p⁴), which introduces electron-electron repulsion within the 2p orbital.
  • 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
   • A significant jump in ionization energy occurs when removing an electron from a lower principal energy level (e.g., from 2p to 1s).
   • This indicates that electrons are organized into distinct energy levels and sublevels.
2. Discontinuities Within a Period
   • 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).
   • This supports the idea that sublevels have different energy levels and stability.
3. Transition Metals and d Sublevels
   • 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.
   • This provides further evidence for the existence of d sublevels.

Reflection