3.1.3Additional lessons
Periodic Trends — IB Chemistry HL Revision Notes & Topical Questions
5 min read 4 parts Lesson 50 of 172
Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
Section 1 of 4Across a Period
Atomic radius
- Atomic radius Radius decreases because nuclear charge rises while electrons enter the same shell with similar shielding, pulling the shell inward.
Ionisation energy
- Ionisation energy The first ionisation energy generally increases because the outer electron feels a greater effective nuclear charge.
Electron affinity
- Electron affinity Atoms attract an added electron more strongly towards the right, so the magnitude of electron affinity generally rises.
Electronegativity
- Electronegativity Electronegativity increases because the nucleus attracts the bonding pair more strongly in smaller atoms with higher nuclear charge.
Metallic character
- Metallic character Elements shift from metallic on the left to non-metallic on the right, as ease of losing electrons falls.
Section 2 of 4Down a Group
1
Each step down adds a new occupied energy level, so atomic radius increases markedly.
2
Inner shells shield the outer electrons, so effective nuclear charge changes little while distance grows.
3
The outer electron is therefore held less strongly and first ionisation energy decreases down a group.
4
Electronegativity also falls because the bonding pair is further from the nucleus and more shielded.
5
Metallic character increases, which is why reactivity of Group 1 rises but reactivity of Group 17 falls.
| Property | Across period | Down group |
|---|---|---|
| Atomic radius | Decreases | Increases |
| First ionisation energy | Increases | Decreases |
| Electronegativity | Increases | Decreases |
| Metallic character | Decreases | Increases |
Section 3 of 4Ions and Exceptions
Ions and Exceptions
Cation size
- Cation size A cation is smaller than its atom because it has lost its outer shell, or has fewer electrons repelling each other.
Anion size
- Anion size An anion is larger than its atom because extra electrons increase repulsion while nuclear charge stays constant.
Dip: Be to B
- Dip: Be to B Boron's first ionisation energy is lower than beryllium's because its outer electron is in a higher-energy 2p sub-level.
Dip: N to O
- Dip: N to O Oxygen's value is lower than nitrogen's because a paired 2p electron experiences repulsion from its partner.
Section 4 of 4Exam Checklist & Tips
3.1.3 – Key Points for the Exam
- Effective nuclear charge The net attraction felt by an outer electron after allowing for shielding by inner electrons.
- Shielding Repulsion of outer electrons by inner electrons, reducing the pull from the nucleus.
- First ionisation energy Energy to remove one mole of electrons from one mole of gaseous atoms.
- Electronegativity The attraction of an atom for the bonding pair of electrons in a covalent bond.
Common Mistakes
- Explaining trends with 'more protons' without mentioning shielding or distance.
- Claiming that atomic radius increases across a period.
- Saying noble gases have high electronegativity because they have full shells.
- Forgetting that the anomalies between groups 2 and 13 and between 15 and 16 exist.
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