What is chemical periodicity and how does it relate to the periodic table?

Chemical periodicity refers to the recurring trends that are observed in the properties of elements as you move across or down the periodic table. These trends arise due to the periodic nature of the electronic configurations of the elements. In the context of the Cambridge International A Levels, understanding these trends helps predict the behavior of elements in chemical reactions.

How does atomic radius change across a period and down a group?

As you move across a period from left to right, the atomic radius generally decreases. This is because the increase in nuclear charge pulls the electrons closer to the nucleus. Conversely, as you move down a group, the atomic radius increases due to the addition of electron shells, which outweighs the increase in nuclear charge.

What is the trend in ionization energy across periods and down groups?

Ionization energy generally increases across a period due to the increasing nuclear charge, which holds the electrons more tightly. However, it decreases down a group because the outer electrons are further from the nucleus and are shielded by inner electron shells, making them easier to remove.

How does electronegativity vary in the periodic table?

Electronegativity tends to increase across a period as the atoms become smaller and the nuclear charge increases, making it easier for the atom to attract electrons. It decreases down a group because the increased distance between the nucleus and the valence electrons reduces the nucleus's ability to attract electrons.

Why do elements in the same group exhibit similar chemical properties?

Elements in the same group have similar chemical properties because they have the same number of valence electrons. This similarity in electron configuration leads to similar behavior in chemical reactions, which is a key concept in understanding chemical periodicity in the context of the Cambridge International A Levels Chemistry curriculum.

Why is "Identifying an element from one clue only." a common mistake in Chemical periodicity of other elements?

Why it happens: Jumping to a conclusion. How to avoid it: Combine clues: oxide acid/base nature + chloride behaviour + reaction with water. Source: 9701 Examiner Reports 2022-2024

Why is "Misreading the group from the oxide formula." a common mistake in Chemical periodicity of other elements?

Why it happens: Not using the oxidation number. How to avoid it: The element's oxidation number in its highest oxide equals its group number.

Why is "Assuming other periods behave exactly like Period 3." a common mistake in Chemical periodicity of other elements?

Why it happens: Over-applying the Period 3 pattern. How to avoid it: Trends are similar but shift — e.g. metallic character grows down a group, so oxides become more basic.

The Periodic Table: chemical periodicity(AS-Level Inorganic Chemistry)Cambridge International A Level Chemistry 9701

Chemical periodicity of other elements

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Chemical Periodicity of Other Elements — Cambridge International AS & A Level Chemistry 9701 (2025-2027 syllabus)

How to use the patterns of chemical periodicity to predict the characteristic properties of an element from its group, and to deduce the identity and position in the Periodic Table of an unknown element from given data.

What you’ll learn

Mapped to the Cambridge International A Level 9701 syllabus (2025-2027).

9.3.1 — Predict the characteristic properties of an element in a given group by using knowledge of chemical periodicity.
9.3.2 — Deduce the nature, possible position in the Periodic Table and identity of unknown elements from given information about physical and chemical properties.

Predicting properties from periodicity

Patterns repeat — so an element behaves like the other members of its group and follows the across-period trends.

Chemical periodicity means properties repeat in a regular pattern across the Periodic Table. This lets you predict an unknown element's behaviour:

Same group → similar chemistry. All members of a group have the same number of outer-shell electrons, so they form the same types of compound and react similarly (e.g. all Group 1 metals react with water to give an alkali + H₂).
Across a period: elements change from metallic to non-metallic; their oxides change from basic → amphoteric → acidic; the highest oxidation number rises with group number.
Down a group: for metals, reactivity increases (easier to lose electrons); for non-metals, reactivity decreases.

Worked. Predict the properties of element X in Group 2, Period 4 (calcium): a reactive metal, forms X²⁺, a basic oxide XO that reacts with acids, reacts with water to give XOH₂ + H₂, and is more reactive than magnesium (further down the group).

Same group → similar chemistry (same outer electrons).
Across a period: metallic → non-metallic; oxides basic → acidic.
Down a group: metal reactivity ↑, non-metal reactivity ↓.

Deducing an unknown element's identity and position

Work backwards from the clues — the nature of its oxide and chloride pins down its group and period.

Given data about an unknown element, use these diagnostic clues:

Clue	What it tells you
Oxide is basic (reacts with acids)	a metal (left of the period)
Oxide is acidic (reacts with bases)	a non-metal (right of the period)
Oxide is amphoteric	a metalloid/borderline element (e.g. Al-like)
Chloride hydrolyses / fumes HCl	a covalent chloride → towards the right
Chloride dissolves neutral	an ionic chloride → a reactive metal
Reacts vigorously with water → alkali	Group 1; less vigorous → Group 2
Highest oxidation number in its oxide	the group number

Worked. An element forms an oxide of formula X₂O₃ that is amphoteric, and a chloride that fumes in moist air. Deduce X.

Oxide X₂O₃ → oxidation number +3 → Group 13.
Amphoteric oxide + covalent (hydrolysing) chloride → matches aluminium.

Tip. Combine several clues — one alone is rarely conclusive, but the oxide's acid/base nature plus the chloride's behaviour usually fix the group.

Basic oxide → metal; acidic oxide → non-metal; amphoteric → borderline.
Hydrolysing chloride → covalent → right of period.
Highest oxide oxidation number = group number.
Combine clues to fix group and period.

How it’s examined

This appears as 'predict/deduce' questions on Papers 1 and 2 that test whether you can apply the Period 3 patterns to an unfamiliar element. Examiner reports note candidates relying on a single clue; the strongest answers combine the oxide's acid–base nature with the chloride's behaviour.

Sources: Cambridge International AS & A Level Chemistry 9701 syllabus (2025-2027), section 9.3; 9701 Examiner Reports 2022-2024; 9701/12 May/Jun 2024 question paper and mark scheme. Last reviewed 2026-05-20.

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Step-by-step worked examples — Chemical periodicity of other elements

Step-by-step solutions to past-paper-style questions on chemical periodicity of other elements, written exactly the way a tutor would explain them at the board.

1Predicting a Group 2 element (3 marks)
Getting started• prediction
Question
Strontium is in Group 2, below calcium. Predict three properties.
Step-by-step solution
1. Step 1
  Forms Sr²⁺ (2 outer electrons lost).
2. Step 2
  Basic oxide SrO that reacts with acids; reacts with water to give Sr(OH)₂ + H₂.
3. Step 3
  More reactive than calcium (further down the group, easier to lose electrons).
Answer
Forms Sr²⁺; basic oxide / reacts with water → alkali + H₂; more reactive than Ca.
2Formulae from group number (2 marks)
Getting started• formulae, group number
Question
An element X is in Group 14. Give the formula of its highest oxide and its chloride.
Step-by-step solution
1. Step 1
  Highest oxidation state = group number = +4.
2. Step 2
  Oxide XO₂ and chloride XCl₄.
Answer
Highest oxide XO₂; chloride XCl₄.
Examiner tip
The highest oxidation state usually equals the group number — use it to write oxide/chloride formulae.
3Deducing an unknown element (4 marks)
Building confidence• deduction
Question
Element Z forms an acidic oxide ZO₂ and a chloride ZCl₄ that fumes in moist air and reacts with water to give an acidic solution. Deduce the group of Z and suggest its identity.
Step-by-step solution
1. Step 1
  Oxide ZO₂ → oxidation number +4 → Group 14.
2. Step 2
  Acidic oxide + hydrolysing covalent chloride → a non-metal/metalloid towards the right.
3. Step 3
  Group 14, forms covalent +4 chloride → consistent with silicon (or carbon).
Answer
Group 14; likely silicon (acidic SiO₂, hydrolysing SiCl₄).
Examiner tip
Use the oxide oxidation number for the group and the chloride behaviour for metal/non-metal character.
4Predicting an element by analogy (3 marks)
Building confidence• prediction, analogy
Question
Rubidium is in Group 1, below potassium. Predict its reaction with water and how its reactivity compares with potassium. (3)
Step-by-step solution
1. Step 1
  Forms Rb⁺ and reacts vigorously with water.
  $2\text{Rb} + 2\text{H}_2\text{O} \rightarrow 2\text{RbOH} + \text{H}_2$
2. Step 2
  More reactive than K — further down the group, the outer electron is more easily lost (lower IE).
Answer
2Rb + 2H₂O → 2RbOH + H₂; even more vigorous than potassium (lower ionisation energy).
5Deducing a Group 1 element (4 marks)
Stretch• deduction
Question
Element Q has a basic oxide Q₂O, a chloride QCl that dissolves to a neutral solution, and reacts violently with water to give an alkali and hydrogen. Deduce its group and justify each clue.
Step-by-step solution
1. Step 1
  Q₂O (oxidation number +1) and QCl → Group 1.
2. Step 2
  Basic oxide + ionic chloride dissolving neutral → a reactive metal.
3. Step 3
  Violent reaction with water → alkali + H₂ is the signature of a Group 1 metal.
4. Step 4
  Conclusion: Q is a Group 1 (alkali) metal.
Answer
Group 1: +1 oxide/chloride, ionic neutral chloride, vigorous water reaction → metal + H₂.
Examiner tip
Cross-check ALL clues — each one (oxide ON, chloride pH, water reaction) must agree on the group.
6Metallic character down a group (4 marks)
Stretch• group trend
Question
Down Group 14 from carbon to lead, predict how metallic character and the acid–base nature of the oxides change. (4)
Step-by-step solution
1. Step 1
  Metallic character increases down the group (atoms larger, outer electrons lost more easily).
2. Step 2
  Top (C, Si): non-metals/metalloids with acidic oxides (CO₂, SiO₂).
3. Step 3
  Ge: metalloid with an amphoteric oxide.
4. Step 4
  Bottom (Sn, Pb): metals whose oxides are amphoteric/more basic in character.
Answer
Metallic character rises down the group; oxides shift from acidic (C, Si) through amphoteric (Ge, Sn, Pb).
Examiner tip
Down a group = more metallic = oxides become less acidic / more basic — the same logic as across a period, reversed.

Key Formulae — Chemical periodicity of other elements

The formulae you need to memorise for chemical periodicity of other elements on the Cambridge International A Level 9701 paper, with every variable defined in plain English and a note on when to use it.

Highest oxidation state = group number
$\text{highest ON} = \text{group number}$
$highest ON$
the maximum oxidation number an element shows
When to use
Predicting highest-oxide and chloride formulae from the group (e.g. Group 14 → XO₂, XCl₄).

Key Definitions and Keywords — Chemical periodicity of other elements

Definitions to memorise and the exact keywords mark schemes credit for chemical periodicity of other elements answers — sharpened from recent examiner reports for the 2026 Cambridge International A Level 9701 sitting.

Chemical periodicity
Examiner keyword
The regular, repeating variation in the chemical properties of elements across the Periodic Table.
Group trend (metallic character)
Examiner keyword
Down a group, atoms get larger and lose electrons more easily, so metallic character increases and oxides become less acidic / more basic.
Prediction by analogy
Examiner keyword
Using the known behaviour of an element to predict that of others in the same group or period.

Common Mistakes and Misconceptions — Chemical periodicity of other elements

The traps other students keep falling into on chemical periodicity of other elements questions — taken from recent Cambridge International A Level 9701 examiner reports and mark schemes — and how to avoid them.

✕Identifying an element from one clue only.
9701 Examiner Reports 2022-2024
Why it happens
Jumping to a conclusion.
How to avoid it
Combine clues: oxide acid/base nature + chloride behaviour + reaction with water.
✕Misreading the group from the oxide formula.
Why it happens
Not using the oxidation number.
How to avoid it
The element's oxidation number in its highest oxide equals its group number.
✕Assuming other periods behave exactly like Period 3.
Why it happens
Over-applying the Period 3 pattern.
How to avoid it
Trends are similar but shift — e.g. metallic character grows down a group, so oxides become more basic.

Practice questions

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Past paper style quiz

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Chemical periodicity of other elements — frequently asked questions

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Chemical periodicity of other elements

Short Study Notes in the form of Slides

Short Study Notes — Chemical periodicity of other elements

Detailed Notes

What you’ll learn

How it’s examined

1Predicting a Group 2 element (3 marks)

2Formulae from group number (2 marks)

3Deducing an unknown element (4 marks)

4Predicting an element by analogy (3 marks)

5Deducing a Group 1 element (4 marks)

6Metallic character down a group (4 marks)

Highest oxidation state = group number

Chemical periodicity

Group trend (metallic character)

Prediction by analogy

✕Identifying an element from one clue only.

✕Misreading the group from the oxide formula.

✕Assuming other periods behave exactly like Period 3.

Practice questions

Past paper style quiz

Assess your understanding

Chemical periodicity of other elements — frequently asked questions