Notes for S3.1.7 Discontinuities in ionization energy trends (Higher Level Only) - IB | RevisionDojo
S3.1.7 Discontinuities in ionization energy trends (Higher Level Only) Notes
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.
Ionization energy trend across the period.
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.
Trends in ionizations energy between Group 2 and Group 13 elements.
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A hypothetical element X shows the following successive ionization energies (in kJ mol−1):
E_2 = 1400,
E_3 = 2300,
E_4 = 7800.$$ What conclusion about X’s electron configuration does the sharp rise from $E_3$ to $E_4$ support?
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What happens to nuclear charge as you move across a period?
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Note
Ionization Energy and its Discontinuities
Ionization energy is the energy required to remove an electron from an atom in its gaseous state.
It generally increases across a period due to increasing nuclear charge and decreasing atomic radius.
However, there are specific points where this trend is interrupted, known as discontinuities.
AnalogyThink of ionization energy as the effort needed to pull a book from a tightly packed shelf. As the shelf gets more crowded (like electrons in an atom), it generally becomes harder to remove a book. But if you find a naturally organized section (like a half-filled sublevel), it might be easier to pull one out.
