Group I, Group VII and Transition Elements

Chemistry· 8.2, 8.3, 8.4 (2026-2028 syllabus)· 25 min read

1. Group I (Alkali Metals) – Core Properties & Trends★★☆☆☆⏱ 6 min

📘 Definition

Alkali Metals

Group I reactive metals located in the first column of the periodic table, each with 1 valence electron that is lost during reactions to form a +1 cation.

All Group I metals are soft, low density, and have much lower melting points than typical metals. Their reactivity increases as you move down the group, because the single valence electron is further from the positively charged nucleus, so it is lost more easily during reactions.

The core reaction you must memorize is their reaction with cold water: all Group I metals react vigorously to produce a metal hydroxide (which forms an alkaline solution) and hydrogen gas.

📐 Worked Example

Describe all observations when a small piece of lithium is added to cold water, and write the word equation for the reaction.

1
1. Observations: Lithium floats on the water surface, fizzes steadily as hydrogen gas is released, moves slowly across the surface, and eventually dissolves completely. If universal indicator is added to the final solution, it turns dark blue or purple, confirming an alkaline product.
2
1. Word equation: lithium + water → lithium hydroxide + hydrogen

Exam tip:

Always name the specific metal hydroxide (e.g. potassium hydroxide) in your reaction equations, not the generic 'metal hydroxide', to earn full marks.

2. Group VII (Halogens) – Core Properties & Trends★★★☆☆⏱ 7 min

📘 Definition

Halogens

Group VII non-metals located in the seventeenth column of the periodic table, each with 7 valence electrons. They exist as diatomic molecules (e.g. $C l_{2}$ , $B r_{2}$ ) and gain 1 electron during reactions to form a -1 halide ion.

Halogens have distinct states and colours at room temperature and pressure (RTP): fluorine is a pale yellow gas, chlorine is a pale green gas, bromine is a red-brown liquid, and iodine is a purple-grey solid that sublimes to form purple vapour when heated. Melting and boiling points increase down the group, while reactivity decreases down the group.

📐 Worked Example

An unknown Group VII element is a liquid at RTP. Predict its colour, name, and state if it is more or less reactive than fluorine.

1
1. Use the state trend: Melting/boiling points increase down Group VII, so the only liquid halogen at RTP is bromine.
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1. Colour prediction: Bromine is red-brown in colour.
3
1. Reactivity comparison: Reactivity decreases down Group VII, so bromine is less reactive than fluorine.

Exam tip:

You will often be asked to match halogens to their physical states and colours in exam questions, so memorize these four core halogens explicitly.

3. Group VII (Halogens) – Displacement Reactions★★★☆☆⏱ 5 min

A more reactive halogen displaces a less reactive halogen from an aqueous solution of its halide salt. Because reactivity decreases down Group VII, chlorine displaces both bromine and iodine, and bromine displaces iodine. Describing these displacement reactions, writing their word equations, and stating the colour changes are all Core content.

📐 Worked Example

Write the word equation for the reaction between bromine water and potassium iodide solution, and state the expected observation.

1
1. Compare reactivity: Bromine is more reactive than iodine, so it will displace iodine from potassium iodide.
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1. Word equation: bromine + potassium iodide → potassium bromide + iodine
3
1. Observation: The colourless potassium iodide solution turns brown/orange as iodine is formed in solution.

Exam tip:

Displacement reactions, their colour changes and word equations are required at Core tier; only the electron-transfer (redox) explanation and ionic half-equations are Extended.

4. Transition Elements – Core & Extended Properties★★☆☆☆⏱ 5 min

📘 Definition

Transition Elements

Metals located between Group II and Group III of the periodic table, with properties that distinguish them clearly from Group I alkali metals.

Core properties: High melting points, high density, hard and strong, much less reactive than Group I metals (no vigorous reaction with cold water)
Core properties: Form coloured compounds (e.g. copper sulfate is blue, iron(III) oxide is orange-brown), and act as catalysts for many reactions (e.g. iron in the Haber process, nickel in alkene hydrogenation)
Extended only property: Have variable oxidation states, so they can form more than one ion (e.g. $F e^{2 +}$ (iron(II)) and $F e^{3 +}$ (iron(III)) ions)

📐 Worked Example

State three differences between the properties of Group I sodium and transition element copper.

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1. Sodium has a low melting point (98°C), while copper has a very high melting point (1085°C).
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1. Sodium reacts violently with cold water, while copper does not react with cold water at all.
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1. Sodium only forms white/colourless compounds, while copper (a transition element) forms coloured compounds, for example copper(II) compounds are blue or green.

Exam tip:

Exam questions often ask you to compare transition metals to Group I metals, so make a note of the key contrasting properties to earn full marks quickly.

5. Extended: Halogen Displacement as Electron Transfer (Redox)★★★★☆Extended only⏱ 4 min

At Extended tier you must explain a halogen displacement reaction as a redox reaction in terms of electron transfer. The more reactive halogen is reduced: its molecules gain electrons to become halide ions. The less reactive halide ions are oxidised: they lose electrons to form halogen molecules.

📐 Worked Example

For the reaction between chlorine and aqueous potassium iodide, write the ionic equation and identify what is oxidised and what is reduced.

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1. Ionic equation (spectator K⁺ ions omitted): Cl₂ + 2I⁻ → 2Cl⁻ + I₂
2
1. Chlorine is reduced: each Cl₂ molecule gains 2 electrons to form 2Cl⁻ ions.
3
1. Iodide is oxidised: two I⁻ ions lose one electron each to form one I₂ molecule.

6. Common Pitfalls

Wrong move:

Stating that Group I reactivity decreases down the group

Why:

Students mix up Group I and Group VII reactivity trends

Correct move:

Recall Group I reactivity increases down the group (valence electron lost more easily), Group VII reactivity decreases down the group (electron gained less easily)

Wrong move:

Writing the product of Group I + water as metal oxide instead of metal hydroxide

Why:

Confusing the reaction of metals with water vs reaction with oxygen

Correct move:

All Group I + cold water reactions produce a metal hydroxide (alkaline product) and hydrogen gas, no oxide is formed

Wrong move:

Stating halogens are monatomic at RTP

Why:

Confusing halogens with noble gases (Group 0)

Correct move:

All halogens exist as diatomic molecules ( $X_{2}$ ) at standard room temperature and pressure

Wrong move:

Claiming transition elements have no catalytic uses

Why:

Forgetting a core characteristic property of transition elements

Correct move:

Transition elements and their compounds are widely used as industrial catalysts, e.g. iron for the Haber process, manganese(IV) oxide for hydrogen peroxide decomposition

Wrong move:

Stating all transition metal ions have a +2 oxidation state (Extended)

Why:

Forgetting the variable oxidation state property of transition elements

Correct move:

Transition elements can have multiple oxidation states, e.g. iron forms both $F e^{2 +}$ and $F e^{3 +}$ ions

7. Quick Reference Cheatsheet

Group/Class	Core Property 1	Core Property 2	Key Reaction/Use
Group I (Alkali Metals)	1 valence electron, forms +1 ion	Reactivity increases down group	Reacts with cold water → metal hydroxide + hydrogen
Group VII (Halogens)	7 valence electrons, forms -1 ion, diatomic	Reactivity decreases down group	More reactive halogen displaces less reactive from halide salt (Extended)
Transition Elements	High melting point, high density, low reactivity	Coloured compounds, catalytic properties	Variable oxidation states (Extended), used as industrial catalysts

8. Frequently Asked

Do all Group 1 metals float on water?

Only the first three (lithium, sodium, potassium) float; rubidium and caesium are denser than water and sink.

Are transition elements more reactive than Group 1 metals?

No, Group 1 metals are far more reactive, especially lower down the group, while most transition metals do not react vigorously with water or oxygen at room temperature.

What's Next

Now that you have mastered the properties of Group I, Group VII and transition elements, you are ready to build your full understanding of the Periodic Table unit with Group 0 noble gases, and extended redox reaction content that underpins halogen displacement reactions. This topic is heavily tested in both multiple-choice (Paper 1/2) and structured question (Paper 3/4) sections of the CIE IGCSE Chemistry 0620 exam, so make sure you memorize the key trends and practice writing reaction equations correctly. We recommend working through past paper questions focused on these groups to familiarize yourself with common exam phrasing and marking scheme requirements.