Periodic Trends
A concise revision guide to periodicity: atomic radius and ionisation energy trends across a period and down a group, melting and boiling point trends across Period 3, and how physical properties link to bonding and structure for Cambridge International AS & A Level Chemistry (9701).
GCSE Recap: Groups, Periods and Outer Electrons
Three quick GCSE questions on the layout of the periodic table before you meet the trends that layout produces.
What Is Periodicity?
Periodicity refers to the repeating pattern of chemical and physical properties seen across different periods of the periodic table. The same pattern appears in Period 2 and Period 3 because electrons are added to the same types of subshells in the same sequence.
The 9701 specification requires you to understand periodicity in terms of a repeating pattern, and to explain trends in melting and boiling temperatures, ionisation energy and atomic radius across periods 2 and 3.
Key idea: periodic trends repeat because the electronic structure of each period follows the same pattern of subshell filling.
Atomic Radius Across a Period
Atomic radius generally decreases across a period. As proton number increases, nuclear charge increases. Electrons are added to the same main shell, so shielding remains similar. The stronger nuclear attraction pulls the electron cloud closer to the nucleus.
Nuclear charge increases
More protons in the nucleus attract the outer electrons more strongly, pulling them closer.
Shielding stays similar
Electrons enter the same main shell, so no significant extra inner-shell shielding is added.
Atomic radius decreases
The stronger attraction reduces the distance between the nucleus and the outer electrons.
Quick Check: Atomic Radius in Period 4
Take the reasoning of this card into a period the page has not used.
Across a period, atomic radius decreases because nuclear charge increases while shielding remains similar.
Ionisation Energy Compared Across a Period and Down a Group
It is useful to separate the two directions clearly. Across a period, electrons are added to the same main shell, so shielding remains similar and atomic radius decreases. First ionisation energy generally increases.
Down a group, electrons are added to new shells. This gives a larger atomic radius and more shielding, so first ionisation energy decreases.
| Direction | Atomic radius | Shielding | First ionisation energy |
|---|---|---|---|
| Across a period (left to right) | Decreases | Similar | Generally increases |
| Down a group | Increases | Increases | Decreases |
Quick Check: Going Down a Group
Build the explanation for the direction the page gives no data for.
Quick Check: Five Pairs of Elements
Decide each pair quickly, using both directions of the table.
Melting and Boiling Point Trends Across Period 3
Across Period 3, melting and boiling points do not follow the same pattern as ionisation energy. This is because melting and boiling points depend on bonding and structure, not on how tightly electrons are held.
Sodium, magnesium and aluminium are metallic solids with giant metallic structures. Silicon has a giant covalent structure with very strong bonds throughout. Phosphorus, sulfur, chlorine and argon are simple molecular or monatomic, so their melting and boiling points are low and depend on weak intermolecular forces.
Exam focus: do not explain melting point trends using ionisation energy arguments. Melting point depends on the type of bonding and structure, not on the energy needed to remove an electron.
Quick Check: Order a Period by Melting Temperature
Rank five Period 2 elements by structure, lowest melting temperature first.
Physical property trends depend on bonding and structure, so they should not be explained using ionisation energy arguments alone.
Using Data to Illustrate Periodicity
The 9701 specification asks you to illustrate periodicity using data, including electronic configurations, atomic radii, melting and boiling temperatures and first ionisation energies (spec point 26).
When answering these questions, state the trend clearly, identify the direction (across a period or down a group) and explain the cause using nuclear charge, shielding, atomic radius and the type of bonding or structure.
Electronic configuration
Repeating pattern of subshell filling from s to p explains why properties repeat across periods.
Atomic radius
Decreases across a period (nuclear charge dominates), increases down a group (extra shells).
Melting and boiling temperatures
Determined by structure: metallic, giant covalent, or simple molecular/monatomic.
First ionisation energies
Generally increase across a period; decrease down a group. Small exceptions at Gp2-Gp3 and Gp5-Gp6 junctions.
Quick Check: Matching the Argument to the Trend
Pick the one accurate statement in each round to finish the page.
Electrical Conductivity Across Period 3
Cambridge 9.1 asks for the periodicity of electrical conductivity as well as melting point, both explained by structure and bonding.
| Element | Na | Mg | Al | Si | P | S | Cl | Ar |
|---|---|---|---|---|---|---|---|---|
| Structure | Giant metallic | Giant metallic | Giant metallic | Giant covalent | Simple molecular P4 | Simple molecular S8 | Simple molecular Cl2 | Monatomic |
| Conductivity | Good | Better | Best | Semiconductor (poor) | None | None | None | None |
The metals conduct because each atom releases its outer electrons into a sea of delocalised electrons that can move through the lattice under a potential difference. Conductivity rises from sodium to aluminium: sodium gives one electron per atom, magnesium two and aluminium three, so aluminium has the highest density of mobile charge carriers.
Silicon is a semiconductor. In its giant covalent lattice every outer electron is held in a localised covalent bond, so conductivity is very low, though a few electrons can be freed by heat.
The non-metals do not conduct. Phosphorus, sulfur and chlorine exist as separate molecules with all electrons in bonds or lone pairs, and argon is single atoms; there are no delocalised electrons and no ions, so no charge can move.
Electrical conductivity and structure across Period 3: rising for the three metals, very low for silicon, zero for the molecular non-metals and argon
Conductivity across Period 3: only the metals have delocalised electrons to carry charge.
Exam sentence: Sodium, magnesium and aluminium conduct because they have delocalised electrons, and the conductivity increases with the number of outer electrons each atom delocalises; silicon, phosphorus, sulfur, chlorine and argon have no delocalised electrons and do not conduct.
Quick Check: Conducting When Hot
Explain why heat helps one Period 3 element to conduct and hinders another.
FAQs: Periodic Trends
Use these questions to secure the key reasoning for periodic trend exam answers.
What is periodicity?
Periodicity is the repeating pattern of physical and chemical properties seen across different periods of the periodic table, caused by the repeated pattern of subshell filling.
Why does atomic radius decrease across a period?
Nuclear charge increases while electrons are added to the same main shell, so shielding remains similar and the stronger nuclear attraction pulls the electron cloud closer to the nucleus.
Why do melting and boiling points not follow the same trend as ionisation energy across a period?
Melting and boiling points depend on bonding and structure. Metallic elements have giant metallic structures, silicon has a giant covalent structure, and the other Period 3 elements are simple molecular or monatomic with weak intermolecular forces.
Why does first ionisation energy generally increase across a period?
As proton number increases, nuclear charge increases but shielding remains similar because electrons enter the same main shell. The stronger attraction between the nucleus and the outer electron means more energy is needed to remove it.
Why does first ionisation energy decrease down a group?
Going down a group, atoms have more occupied electron shells. The outer electron is further from the nucleus and shielded more strongly by inner shells, so less energy is needed to remove it.
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Some ionic radii are shown.
| Ion | Ionic radius / nm |
|---|---|
| Na+ | 0.102 |
| K+ | 0.138 |
| F− | 0.133 |
| Cl− | 0.180 |
Which compound has the strongest ionic bonding?
Explain why the metallic bonding in magnesium is much stronger than that in sodium.
Copyright notice: This OLS revision content, including the explanations, layout, diagrams, tables and embedded learning structure, is authored for Online Learning System by Dr. Mohammed Al-Fatah. It may not be copied, reproduced, redistributed or adapted without written permission.
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