Periodicity, Periodic Trend in Electron Configuration and Ionisation Energy
Quick Notes
- Ionisation energy:
- First ionisation energy: Energy to remove 1 mol of electrons from 1 mol of gaseous atoms.
- Increases across a period (increased nuclear charge, decreased atomic radius).
- Decreases down a group (increased shielding, increased atomic radius).
- Anomalies: Group II to Group III (electron removed from p sub-shell); Group V to Group VI (electron removed from paired orbital, electron pair repulsion).
- Successive ionisation energies show shell structure.
- Electron configuration trends:
- Periods 2 and 3 show gradual filling of s- and p-orbitals.
Full Notes
Electron Configuration and Block Classification
Across Periods 2 and 3, electrons fill sub-shells in a regular pattern:
- Period 2: 2s to 2p
- Period 3: 3s to 3p
Elements are classified into blocks based on their outer sub-shell:
- s-block: Groups 1 and 2 (e.g. Na, Mg)
- p-block: Groups 13–18 (e.g. Cl, Ar)
- d-block: Transition metals (begin filling 3d after 4s)
Ionisation Energy
First Ionisation Energy is the energy required to remove 1 mole of electrons from 1 mole of gaseous atoms:
X(g) → X+(g) + e−
Trends Across a Period
First ionisation energy generally increases across the period.
Reason for the trend:
- Nuclear charge increases (more protons in nucleus).
- Atomic radius decreases, meaning outer electrons are closer to the nucleus.
- Outer electrons are more strongly attracted to nucleus, requiring more energy to remove.
Exceptions to the Trend
- Aluminium (Al) has a lower ionisation energy than Mg.
- Reason: The outer electron in Al is in the 3p subshell, which is higher in energy than the outer electron in Mg (3s) and easier to remove.
- Sulfur (S) has a lower ionisation energy than phosphorus (P).
- Reason: In sulfur, the 3p orbital starts to pair electrons, causing electron pair repulsion, making it easier to remove an electron.
Trends Down a Group
First ionisation energy decreases down a group.
This is due to increased atomic radius and more shielding (despite increased nuclear charge).
Example Down Group 2 (Be to Ba)
1st ionisation energy decreases because:
- Atomic radius increases
- Shielding increases (more inner electron shells)
- Outer electrons are further from the nucleus and experience weaker attraction
Successive Ionisation Energies
Large jumps in successive ionisation energies for an element indicate an electron being removed from a new, inner shell.
This helps determine the number of electrons in each shell and thus, the element's group.
Summary
- Electron configuration shows regular filling of orbitals across periods.
- First ionisation energy increases across a period due to increased nuclear charge and smaller radius.
- Decreases down a group due to increased shielding and atomic radius.
- Anomalies occur at Group III and Group VI elements.
- Successive ionisation energies reveal electron shell structure.