Lesson 3 of 3

Groups and deduction

Apply H1 Group 1/17 trends to new evidence.

A-Level 8873, revised syllabus (2026-2027)

I can connect Period 3 compounds to their behaviour.Syllabus
Syllabus 8873, 4(e). Connect Period 3 compounds to their behaviour.
I can compare reducing and oxidising behaviour.Syllabus
Syllabus 8873, 4(f). Compare reducing and oxidising behaviour.
I can explain hydrogen-halide thermal stability.Syllabus
Syllabus 8873, 4(g). Explain hydrogen-halide thermal stability.
I can predict properties from group membership.Syllabus
Syllabus 8873, 4(h). Predict properties from group membership.
I can deduce unknown element identity/position.Syllabus
Syllabus 8873, 4(i). Deduce unknown element identity/position.

Separate losing electrons from gaining them

Group 1 metals reduce other species; halogens oxidise them.

Make a guess. You are not marked.

Chlorine water is added to colourless aqueous potassium iodide. What do you see?

A white precipitate of KCl forms
No change, because both are halogens
The solution turns brown as iodine forms

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The solution turns brown as iodine forms

Chlorine is a stronger oxidising agent than iodine, so it oxidises iodide to iodine. Iodine in water is brown.

A reducing agent supplies electrons and is itself oxidised. Group 1 atoms have one outer electron: M → M⁺ + e^-. From lithium through sodium, potassium, rubidium and caesium, the outer electron is farther from the nucleus and more shielded. It is generally lost more easily, so the familiar chemical reactivity increases down the group.

The required Group 1 range
Element	Outer configuration	Shared behaviour
Li	2s¹	Forms a 1+ ion by losing the outer electron
Na	3s¹	Same outer pattern
K	4s¹	Same outer pattern
Rb	5s¹	Same outer pattern
Cs	6s¹	Same outer pattern

A common reaction is 2M(s) + 2H₂O(l) → 2MOH(aq) + H₂(g). The metal is oxidised and water is reduced. For this H1 comparison, explain the trend through ease of electron loss; do not import an H2 electrode-potential ranking, which includes additional energetic factors.

An oxidising agent accepts electrons and is itself reduced. X₂ + 2e^- → 2X^-. From chlorine to bromine to iodine, larger size and greater shielding weaken attraction for an incoming electron. Oxidising ability decreases in the order Cl₂ > Br₂ > I₂.

Check your understanding

Which mixture shows no displacement reaction?

Chlorine water added to aqueous bromide ions
Iodine solution added to aqueous chloride ions
Bromine water added to aqueous iodide ions

Show the answer

Iodine solution added to aqueous chloride ions

Iodine is too weak an oxidising agent to oxidise Cl^- to Cl₂, so nothing happens. Oxidising power falls down Group 17: Cl₂ > Br₂ > I₂.

Thermal stability of the hydrogen halides is a different comparison. HCl is more thermally stable than HBr, which is more stable than HI. Down the group, the halogen atom becomes larger, the H-X bond longer and overlap poorer, so bond energy decreases. Less energy is needed to decompose HI: 2HI(g) ⇌ H₂(g) + I₂(g). This concerns covalent H-X bond breaking, not the intermolecular forces used to explain halogen volatility.

Check your understanding

Iodine has a higher boiling point than chlorine, yet HI is less thermally stable than HCl. Why?

HI forms hydrogen bonds, so it stays together when heated but its molecules split easily.
Boiling overcomes forces between molecules; decomposing HX breaks the H-X bond itself.
Iodine has stronger I-I bonds, so it boils higher, and the same strength weakens H-I.

Show the answer

Boiling overcomes forces between molecules; decomposing HX breaks the H-X bond itself.

I₂ has a larger, more polarisable electron cloud, so stronger forces between molecules. The H-I bond is longer and weaker than H-Cl, so HI decomposes more easily.

Use several clues to identify an unknown

Predict with a periodic pattern, then check the observations.

Worked exampleIdentify a Period 3 element

Element X conducts electricity as a solid. Its highest oxide contains X in oxidation state +3 and reacts with both acid and NaOH. Identify X.

Try it first. Then show the worked stepsHide the worked steps

Conductivity suggests a metal among Na, Mg and Al.
The +3 highest oxide selects aluminium from those candidates.
The oxide amphoterism agrees with Al₂O₃, checking the first inference.

Answer

X is aluminium. State the independent clues rather than guessing from just one property.

Worked examplePredict an unfamiliar group member

An unfamiliar element has outer configuration ns2 np5 and lies below chlorine. Predict two properties relative to chlorine.

Try it first. Then show the worked stepsHide the worked steps

Seven outer electrons place it in Group 17.
A higher occupied shell gives greater size and shielding, so it attracts an incoming electron less strongly.
A larger molecular electron cloud gives stronger instantaneous dipole-induced dipole attractions.

Answer

It is a weaker oxidising agent and its diatomic molecules are less volatile than chlorine. These are separate electronic and intermolecular explanations.

When inferring bonding from an oxide or chloride, combine melting behaviour, conductivity in different states and chemical reactions with water. High melting point does not uniquely establish ionic bonding: SiO₂ is a covalent network. An acidic chloride solution also does not uniquely establish molecular hydrolysis: hydrated metal ions can release protons.

Worked exampleUse an oxide and a chloride to test the same structural model

A Period 3 element forms an oxide that has a very high melting point, does not conduct when molten and does not react with water. Its chloride is a volatile liquid that reacts with water to produce an acidic solution and a white insoluble oxide. Suggest the element and both structures.

Try it first. Then show the worked stepsHide the worked steps

The oxide has strong bonding throughout its structure, because melting needs much energy. Its non-conducting melt argues against an ordinary ionic lattice: mobile ions would carry charge.
A giant covalent oxide fits these observations. SiO₂ is the specified Period 3 example; its extended Si-O network also explains its failure to dissolve in water.
The chloride is volatile, so the attractions separating its particles are relatively weak. A simple molecular chloride fits; giant ionic chlorides such as NaCl have high melting points.
SiCl₄ hydrolysis gives insoluble SiO₂ and HCl, agreeing with both observed products. AlCl₃ can also give acidic water, but that single observation cannot explain the entire set.

Answer

The evidence supports silicon: giant covalent SiO₂ and simple molecular SiCl₄. Bonding is inferred from several independent observations, then checked against the reaction products.

Exam-style question

Paper 2 Section B style
This question draws on Atomic Structure and the Periodic Table.
Q is an element in Period 3. Its first four ionisation energies are shown below.
Table 1. Successive ionisation energies of Q
ionisation 1st 2nd 3rd 4th
energy / kJ mol^-1 577 1820 2740 11600
The first ionisation energy of magnesium is 736 kJ mol^-1.
(a) Define first ionisation energy. [1]
(b) Explain why each successive ionisation energy of Q is larger than the one before. [2]
(c) Use Table 1 to deduce the group of Q. Explain your answer. [2]
(d) Write the full electronic configurations of a Q atom and of the Q³⁺ ion. [2]
(e) Q has a larger nuclear charge than magnesium, yet its first ionisation energy is lower. Explain why. [2]
(f) Explain why atomic radius decreases from sodium to chlorine. [2]
(g) Describe how sodium oxide and phosphorus(V) oxide react with water. Give an equation for each and the approximate pH of the solution formed. [4]
(h) The oxide of Q is amphoteric. Write ionic equations for its reactions with aqueous acid and with aqueous sodium hydroxide. [2]
(i) Element R is also in Period 3. Its chloride is a volatile liquid that reacts with water to give a white solid and an acidic solution. Its oxide has a very high melting point. Identify R, write the equation for the reaction of its chloride with water, and explain the high melting point of its oxide. [3]
[20]

Table 1. Successive ionisation energies of Q
ionisation	1st	2nd	3rd	4th
energy / kJ mol^-1	577	1820	2740	11600

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