Period – IGCSE Chemistry Definition
IGCSE Chemistry definition of period: a horizontal row in the periodic table. Covers trends, electron shells, and exam tips for Cambridge 0620.
Published by IGCSEChemistry.com.my
Chemistry teaching team: K. S. Tan (15+ years teaching IGCSE Chemistry) and Ms Yash (10+ years teaching IGCSE Chemistry) and Ms Kartini (15+ years teaching IGCSE Chemistry).
Mapped to Cambridge IGCSE Chemistry 0620 (2026–2028). Last updated 2026-08-19.
The period concept links directly to electron configuration. Understanding that the period number equals the number of electron shells unlocks many exam questions about element placement, atomic radius, and metallic/non-metallic character.
The 0620 definition
A period is a horizontal row of elements in the periodic table. Elements in the same period have the same number of occupied electron shells.
Periods and electron shells
| Period | Number of shells | Elements | Example |
|---|---|---|---|
| 1 | 1 | H, He | H: 1 electron |
| 2 | 2 | Li to Ne | C: 2, 4 |
| 3 | 3 | Na to Ar | Cl: 2, 8, 7 |
Across a period, the number of outer-shell electrons increases from 1 (Group I) to a full shell (Group 0). This is why the periodic table has its characteristic shape.
Trends across Period 3
| Property | Left → Right trend | Explanation |
|---|---|---|
| Atomic radius | Decreases | More protons pull electrons closer; same number of shells |
| Metallic character | Decreases | Na, Mg, Al are metals; Si is a metalloid; P, S, Cl, Ar are non-metals |
| Oxide type | Basic → amphoteric → acidic | Na₂O basic; Al₂O₃ amphoteric; SO₃ acidic |
| Ionisation | Lose electrons → gain electrons | Metals form +ions; non-metals form −ions |
Metal to non-metal transition
Moving left to right across any period:
Na (metal) → Mg (metal) → Al (metal) → Si (metalloid) → P (non-metal) → S (non-metal) → Cl (non-metal) → Ar (noble gas)
This transition from metallic to non-metallic is a Supplement point that examiners like to test through oxide reactions.
Worked exam question
Sodium (Na) and chlorine (Cl) are both in Period 3. (a) State what is meant by a period. [1] (b) State the number of electron shells in a sodium atom. [1] (c) Explain why sodium is a metal but chlorine is a non-metal. [2]
Mark scheme
(a) A horizontal row (of elements) in the periodic table [1]
(b) 3 [1]
(c) Sodium has 1 outer electron and loses it to form a positive ion / Na⁺ [1]; chlorine has 7 outer electrons and gains 1 to form a negative ion / Cl⁻ [1] — metals lose electrons, non-metals gain electrons
Common exam mistakes
- Confusing period (horizontal row) with group (vertical column).
- Saying period number = number of electrons (it is the number of occupied electron shells).
- Forgetting that atomic radius decreases across a period despite adding more electrons — the increase in nuclear charge pulls all electrons closer.
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