The Periodic Table: Groups and Trends

This guide covers topic 8, The Periodic Table — subtopics 8.1 to 8.5 — for Cambridge IGCSE Chemistry 0620 and Cambridge O Level Chemistry 5070, 2026–2028 series.

A note on group names. Cambridge’s own syllabus wording uses the older, Roman-numeral group names throughout: Group I (not Group 1), Group VII (not Group 17), and Group VIII for the noble gases (not Group 0/18). This guide uses the syllabus’s own terms because that’s the language your exam paper will use — if you’ve seen modern IUPAC numbering elsewhere, Group I, VII and VIII here correspond to modern Groups 1, 17 and 18.

8.1 Arrangement of elements

CORE (0620) · REQUIRED (5070) — describe the Periodic Table as elements arranged in periods and groups, in order of increasing proton (atomic) number; describe the change from metallic to non-metallic character across a period; describe how group number relates to the charge of the ions elements in that group form; explain, using electronic configuration, why elements in the same group share similar chemical properties; explain how an element’s position can be used to predict its properties.

EXTENDED / SUPPLEMENT (0620) · REQUIRED (5070) — given information about elements, identify trends in groups you haven’t necessarily studied by name.

Elements in the same group have the same number of electrons in their outer shell, which is why they react in similar ways — this single fact underlies almost everything else on this page. Elements in the same group form ions of the same charge for the same reason: Group I metals form 1+ ions, Group VII non-metals form 1− ions, and so on.

8.2 Group I properties — the alkali metals

CORE (0620) · REQUIRED (5070) — describe lithium, sodium and potassium as relatively soft metals, with trends down the group of decreasing melting point, increasing density, and increasing reactivity; given information, predict the properties of other Group I elements.

There is no Extended-only content in this subtopic — everything here is Core for 0620 and required for every 5070 candidate alike.

Reactivity increases down Group I because the single outer-shell electron is progressively further from the nucleus and more weakly held as atoms get larger, so it’s lost more easily in reactions. All three metals react vigorously with cold water, releasing hydrogen gas and forming an alkaline hydroxide solution — and the vigour of that reaction is itself the clearest evidence of the reactivity trend, from lithium (steady fizzing) to sodium (fizzes and may melt into a ball) to potassium (ignites the hydrogen released).

8.3 Group VII properties — the halogens

CORE (0620) · REQUIRED (5070) — describe chlorine, bromine and iodine as diatomic non-metals, with trends down the group of increasing density and decreasing reactivity; state their appearance at room temperature and pressure; describe and explain displacement reactions of halogens with other halide ions; given information, predict the properties of other Group VII elements.

Also no Extended-only content — this whole subtopic is Core for 0620 and required for every 5070 candidate.

Halogen	Appearance at r.t.p.
Chlorine, Cl₂	Pale yellow-green gas
Bromine, Br₂	Red-brown liquid
Iodine, I₂	Grey-black solid

A more reactive halogen displaces a less reactive one from a solution of its halide salt:

Cl2(aq) + 2KI(aq) → 2KCl(aq) + I2(aq)

Chlorine is more reactive than iodine, so it displaces iodide ions, forming iodine (visible as the solution turns brown/orange). This links directly to the silver nitrate anion tests — the same reactivity order (Cl > Br > I) explains why chloride, bromide and iodide give progressively less soluble, differently coloured silver halide precipitates.

8.4 Transition elements

CORE (0620) · REQUIRED (5070) — describe the transition elements as metals with high densities, high melting points, that form coloured compounds, and often act as catalysts, both as elements and within compounds.

EXTENDED / SUPPLEMENT (0620) · REQUIRED (5070) — describe transition elements as having ions with variable oxidation numbers, for example iron(II) and iron(III).

Iron is the example the syllabus names directly: iron(II) compounds (like iron(II) sulfate) and iron(III) compounds (like iron(III) chloride) are both stable, ordinary compounds — the metal simply forms two different, equally valid ions. This is unlike Group I metals, which only ever form one ion charge, and it’s exactly the property tested by the iron(II)/iron(III) precipitate colours in qualitative analysis.

8.5 Noble gases — Group VIII

CORE (0620) · REQUIRED (5070) — describe the Group VIII noble gases as unreactive, monatomic gases, and explain this using electronic configuration.

No Extended-only content here either. Noble gases are unreactive because their outer electron shell is already full — there’s no tendency to gain, lose or share electrons, which is precisely the driving force behind bonding for every other group. “Monatomic” means they exist as single, separate atoms rather than as molecules (unlike Group VII, which is diatomic) — there’s simply no bonding pulling their atoms together.

Common mistakes

• Explaining group trends without referring to electronic configuration. “They’re in the same group” is not an explanation on its own — the shared number of outer-shell electrons is.
• Getting the Group I and Group VII reactivity trends backwards. Reactivity increases down Group I (electrons more easily lost) but decreases down Group VII (electrons less easily gained) — the trends run in opposite directions for a consistent reason: distance from the nucleus.
• Forgetting a displacement reaction needs a more reactive halogen and a less reactive halide. Iodine will not displace chloride — the reaction only goes one way.
• Mixing up Cambridge’s Group I/VII/VIII naming with modern Group 1/17/18 numbering on an answer sheet that expects the syllabus’s own terms.
• Assuming every transition metal example is examinable. Iron(II)/iron(III) is the syllabus’s own example; don’t assume others (like copper(I)/copper(II)) are required unless a question gives you the information.

Quick revision checklist

All candidates (0620 Core and all 5070): Periodic Table arrangement by proton number · metallic-to-non-metallic trend across a period · group number and ion charge · Group I trends (melting point, density, reactivity) · Group VII trends (density, reactivity, appearance) · halogen displacement reactions · transition elements as high-density, high-melting-point, coloured, catalytic metals · noble gases as unreactive and monatomic

0620 Extended and all 5070 candidates, additionally: identifying trends in unfamiliar groups from given data · transition elements having variable oxidation numbers (iron(II)/iron(III))

Related resources

• Identification of Ions and Gases — halide precipitate colours and iron(II)/iron(III) tests
• Atoms, Elements and Compounds — electronic configuration behind every trend on this page
• Ionic, Covalent and Metallic Bonding — how group number sets ionic charge
• Group 2: The Alkaline Earth Metals · Group 17: The Halogens — the AS Level continuation of Group I/VIII and Group VII (9701), not required for IGCSE or O Level
• Cambridge IGCSE Chemistry hub · Cambridge O Level Chemistry hub

Written against Cambridge IGCSE Chemistry 0620 and Cambridge O Level Chemistry 5070, 2026–2028 series. Always check the current syllabus for your examination year.