2.1.2Physical Chemistry
Trends of Period 3 Elements: Atomic Radius — AQA AS Chemistry Revision Notes & Topical Questions
5 min read 4 parts Lesson 53 of 102
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.
Part 1 of 4The Trend Across Period 3
Atomic radius decreases steadily from sodium to chlorine.
Nuclear charge
- Nuclear charge The number of protons increases by one for each element across the period, so the positive charge in the nucleus grows.
Same shell
- Same shell Electrons are added to the same outer shell, so the number of full inner shells stays the same.
Shielding
- Shielding Shielding by inner electrons remains roughly constant, because extra electrons are added to the same shell and shield each other poorly.
Net effect
- Net effect Outer electrons are pulled more strongly towards the nucleus, so the atom contracts and the radius decreases.
Part 2 of 4Explaining the Pull
A short logical chain earns full marks on explanation questions.
1
Moving from Na to Cl, each atom has one more proton, so the nuclear charge increases from 11+ to 17+.
2
The extra electron enters the third shell, which means the shielding from inner shells is almost unchanged.
3
The effective nuclear attraction on the outer electrons therefore increases across the period.
4
This stronger attraction draws the electron cloud closer to the nucleus, so the atomic radius gets smaller.
Part 3 of 4Related Trends and Comparisons
Related Trends and Comparisons
Down a group
- Down a group Each new element has an extra occupied shell, so the outer electrons lie further from the nucleus.
Cations versus atoms
- Cations versus atoms A metal ion such as Na⁺ is smaller than its atom because it has lost its entire outer shell.
Anions versus atoms
- Anions versus atoms A non-metal ion such as Cl⁻ is larger than its atom because extra electron repulsion expands the electron cloud.
| Direction | Atomic radius | Main reason |
|---|---|---|
| Across a period (left to right) | Decreases | More protons, same shell and similar shielding |
| Down a group | Increases | Extra electron shell and more shielding |
Part 4 of 4Exam Checklist & Tips
2.1.2 – Key Points for the Exam
- Atomic radius A measure of the size of an atom, related to the distance from the nucleus to the outer electrons.
- Nuclear charge The total positive charge of the protons in the nucleus.
- Shielding The repulsion of outer electrons by inner-shell electrons, which reduces the attraction from the nucleus.
- Electron shell An energy level around the nucleus that holds electrons.
Common Mistakes
- Stating that atoms get smaller because they lose electrons; across a period electrons are added.
- Forgetting to mention that the extra electrons enter the same shell.
- Saying shielding increases across a period, when it stays roughly constant.
- Comparing the size of an ion and its atom without considering electrons lost or gained.
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