How atomic size, metallic character and reactivity change across periods and down groups. Explained by electron shells and shielding.
At a glance
Atomic size: DECREASES across a period; INCREASES down a group.
Across a period: same shells, more protons → stronger pull → smaller atom.
Down a group: more shells, more shielding → outer electrons further out, weaker pull.
Metallic character: DECREASES across a period; INCREASES down a group.
Group 1 reactivity: increases DOWN the group (easier to lose outer e⁻).
Group 17 reactivity: decreases DOWN the group (harder to attract incoming e⁻).
Two-step explanation: (1) shell structure / shielding, (2) effect on electron loss/gain.
What you’ll learn
Mapped to the Cambridge IGCSE 0620 syllabus (2026-2028).
10.2 — Describe trends in atomic radius and reactivity across periods and down groups.
10.2 — Explain trends in terms of electronic structure.
Atomic size (radius)
Across period: smaller. Down group: bigger.
Across a period. Atomic radius DECREASES.
Same number of shells.
Increasing number of protons.
Stronger nuclear pull on the outer electrons.
Outer electrons drawn closer → smaller atom.
Worked. Period 3: Na (∼186pm) > Mg (160) > Al (143) > Si (117) > P (110) > S (104) > Cl (99) > Ar (71).
Down a group. Atomic radius INCREASES.
Adding a new shell each row.
Outer electrons further from the nucleus.
More inner shells SHIELD the outer electrons from the nucleus.
Net pull on outer electrons WEAKER → atom larger.
Worked. Group 1: Li (∼152pm) < Na (186) < K (227) < Rb (248) < Cs (265).
Across a period the nucleus pulls outer electrons closer; down a group each new shell pushes them further out.
Across: more protons → smaller.
Down: more shells + shielding → bigger.
Period 3: Na largest, Ar smallest.
Group 1: Li smallest, Cs largest.
Metallic character
How easily an element loses electrons. Across: less metallic. Down: more metallic.
Metallic character = ease of losing outer-shell electrons (forming positive ions).
Across a period: DECREASES.
Atoms get smaller; outer electrons closer to nucleus.
Pull on outer e⁻ stronger → harder to lose.
More electronegative — easier to GAIN electrons (non-metal behaviour).
Period 3: Na, Mg, Al — metals (lose e⁻). Si — metalloid. P, S, Cl — non-metals (gain e⁻).
Down a group: INCREASES.
Atoms get bigger; outer electrons further from nucleus.
Shielding by inner shells weakens the pull.
Outer e⁻ easier to lose → more metallic.
Group 14: C (non-metal) → Si (metalloid) → Ge (metalloid) → Sn, Pb (metals).
Cambridge tip. When asked "explain why potassium is more metallic than sodium" — say:
K has MORE shells than Na.
Outer e⁻ further from nucleus, more shielded.
Easier to lose.
More metallic.
Always include both shells/shielding AND the consequence (easier to lose).
Metallic = easy to lose e⁻.
Across period: less metallic (smaller atoms).
Down group: more metallic (more shells + shielding).
K more metallic than Na due to more shells.
Group 1 vs Group 17 reactivity
Group 1 reactivity: ↑ down. Group 17: ↓ down. Both explained by atom size.
Group 1 (alkali metals): reactivity INCREASES down the group.
Reactivity = how readily the metal LOSES its 1 outer electron.
Atoms get bigger going down.
Outer e⁻ further from nucleus → weaker pull.
Easier to lose → more reactive.
Order of reactivity: Li < Na < K < Rb < Cs.
Worked qualitative. Caesium reacts EXPLOSIVELY with water (often dropped through the surface). Sodium reacts vigorously (fizzes, sometimes ignites). Lithium reacts more sedately (slow fizz). Same chemistry (2M+2H2O→2MOH+H2) — just different vigour.
Group 17 (halogens): reactivity DECREASES down the group.
Reactivity = how readily the non-metal GAINS 1 electron.
Atoms get bigger going down.
Incoming e⁻ would join an outer shell that's farther from the nucleus, more shielded.
Pull on incoming e⁻ weaker → harder to gain → less reactive.
Order of reactivity: F > Cl > Br > I.
Why opposite directions? For metals (electron LOSS), bigger = better. For non-metals (electron GAIN), bigger = worse. Same physics; different role for the outer shell.
Bigger atoms lose electrons more easily but gain them less easily — so the two groups trend in opposite directions.
Halogen displacement. A more reactive halogen displaces a less reactive halogen from a salt:
Cl2+2KBr→2KCl+Br2.
Cl displaces Br. The brown colour of Br₂ appears.
G1 reactivity ↑ down: bigger atom, easier to lose e⁻.
G17 reactivity ↓ down: bigger atom, harder to gain e⁻.
Same explanation from atom size — opposite consequences.
Halogen displacement reactions follow the trend.
Quick recap
Atomic radius: across ↓, down ↑.
Metallic character: across ↓, down ↑.
G1 reactivity: ↑ down (easier to lose e⁻).
G17 reactivity: ↓ down (harder to gain e⁻).
Always explain via shells + shielding.
Memorise this
Verbatim phrases and definitions Cambridge mark schemes credit.
Atomic radius — distance from nucleus to outer-shell electrons.
Shielding — reduction in nuclear attraction on outer electrons by inner-shell electrons.
Metallic character — tendency of an element to lose electrons and form positive ions.
How it’s examined
Periodic trends are examined every Paper 4 (5-7 marks): explain a trend using shell structure. Examiner reports flag students stopping at 'atom is bigger' without linking to easier electron loss / harder electron gain.
Step-by-step solutions to past-paper-style questions on periodic trends, written exactly the way a tutor would explain them at the board.
1Reactivity trend in Group I
Extended• Adapted from 0620/42 May/Jun 2024 Q11• Group I
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Question
Explain why reactivity INCREASES down Group I.
Step-by-step solution
Step 1
Atoms get LARGER down the group (more shells).
Step 2
Outer electron is FURTHER from the nucleus and SHIELDED by inner shells.
Step 3
Easier to LOSE the outer electron → more reactive.
Answer
Bigger atoms + more shielding → outer electron lost more easily → reactivity increases.
2Reactivity trend in Group VII
Extended• Group VII
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Question
Explain why reactivity DECREASES down Group VII.
Step-by-step solution
Step 1
Atoms get larger down the group → outer shell further from nucleus.
Step 2
Group VII atoms GAIN an electron. Bigger atoms → harder to attract an electron.
Answer
Larger atoms attract incoming electrons less strongly → reactivity decreases.
Examiner tip
Direction of trend depends on whether atoms LOSE (G-I) or GAIN (G-VII) electrons.
3Halogen displacement
Extended• displacement
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Question
What happens when chlorine is bubbled into potassium bromide solution?
Step-by-step solution
Step 1
Cl is more reactive than Br → Cl displaces Br.
Step 2
Solution turns from colourless to brown/orange (Br₂).
Cl2+2KBr→2KCl+Br2
Answer
Solution turns brown/orange as Br2 is displaced.
Key Definitions and Keywords — Periodic Trends
Definitions to memorise and the exact keywords mark schemes credit for periodic trends answers — sharpened from recent examiner reports for the 2026 0620 sitting.
Shielding
Examiner keyword
Inner-shell electrons reduce the effective attraction of the nucleus on outer-shell electrons.
Displacement reaction
Examiner keyword
More reactive element takes the place of a less reactive one in a compound.
Common Mistakes and Misconceptions — Periodic Trends
The traps other students keep falling into on periodic trends questions — taken from recent Cambridge IGCSE 0620 examiner reports and mark schemes — and how to avoid them.
✕Saying reactivity increases down BOTH groups
0620/42 — every series
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Why it happens
Generalising the rule.
How to avoid it
Group I: increases DOWN (lose more easily). Group VII: DECREASES down (gain less easily).
✕Skipping shielding in the explanation
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Why it happens
Forgetting the structural reason.
How to avoid it
Always include: atomic radius CHANGES + SHIELDING + ease of electron transfer.
✕Saying bromine displaces chlorine
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Why it happens
Inverting reactivity.
How to avoid it
More reactive halogen displaces less reactive: F > Cl > Br > I.
Periodic Trends — frequently asked questions
The things students keep getting wrong in this sub-topic, answered.