Discontinuities in Ionization Energy Trends HL Only
Quick Notes
- First ionization energy (IE1) increases across a period due to increasing nuclear charge.
- Unexpected drops (discontinuities) occur between certain elements.
- These discontinuities support the existence of energy sublevels within main shells (s and p).
- The pattern is explained by the relative energy of the electron being removed, not by “special stability”.
Full Notes:
First Ionization Energy Across a Period
First ionization energy increases as a general trend Across a Period.
This is because as you move from left to right across a period:
- Nuclear charge increases
- Atomic radius decreases
- Electron shielding remains relatively constant
But... this trend is not perfectly smooth.
Discontinuities: The Evidence
Key Discontinuities (Using Period 3 as Example)
- Between Mg (Z=12) and Al (Z=13)
Aluminium (Al): Lower first ionization energy than magnesium (Mg), meaning the electron removed must have less attraction to the nucleus (be further away and higher in energy). This is now explained as the outer electron being in a 3p orbital (higher in energy than the 3s orbital in magnesium). - Between P (Z=15) and S (Z=16)
Sulfur (S): Lower first ionization energy than phosphorus (P), meaning the outermost electron removed must be higher in energy and easier to remove. This is explained as due to electron pairing in a 3p orbital, causing repulsion and giving evidence that the 3p subshell must contain 3 orbitals (as the 4th electron in the subshell has to pair up with another electron in an orbital).
What This Tells Us
These drops in ionization energy are not due to irregular nuclear charge, but due to higher energy of p-electrons compared to s-electrons (for group 2 to group 3) and electron repulsion in doubly occupied orbitals (for group 5 to group 6).
This is evidence for energy sublevels (s and p) within a principal energy level (n).
Don't use “Special Stability” as an explanation! While half-filled or full subshells are sometimes called “stable,” IB prefers an explanation based on the actual energy of the orbital and the repulsion forces involved. Focus on the energy changes associated with losing electrons rather than general terms like “stability.”
Summary
- Ionization energy increases across a period but shows discontinuities
- Discontinuities occur due to s vs p orbital energy differences and electron pairing repulsion
- This provides evidence for sublevels within main electron shells
- IB explanations must be based on orbital energy and repulsion, not vague “stability”