Periodicity
A concise revision guide to periodicity: atomic radius and first ionisation energy trends across periods and down groups, melting and boiling point trends across Period 3, and how physical properties link to bonding and structure.
- 3.2.1.2i-a
- 3.2.1.2i-b
- 3.2.1.2i-c
- 3.2.1.2i-d
- 3.2.1.2ii
What these spec points say
- 3.2.1.2i-a describe the trend in atomic radius of the elements Na-Ar
- 3.2.1.2i-b explain the trend in atomic radius of the elements Na-Ar
- 3.2.1.2i-c describe the trend in first ionisation energy of the elements Na-Ar
- 3.2.1.2i-d explain the trend in first ionisation energy of the elements Na-Ar
- 3.2.1.2ii explain the trend in melting point of the elements Na-Ar in terms of their structure and bonding
Period 3 Structures and Melting
Choose a step to watch each Period 3 structure, then see how much energy each element needs to melt.
© Dr. Mohammed Al-Fatah – onlinelearningsystem.net
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 specification requires you to understand periodicity as a repeating pattern of physical and chemical properties, and to explain trends in atomic radius, first ionisation energy, and melting and boiling points using electronic structure, bonding and structure.
Key idea: periodic trends repeat because the electronic structure of each period follows the same pattern of subshell filling.
s, p, d and f Blocks
The Periodic Table lists the elements in order of proton number, and an element is classified as s block, p block, d block or f block according to its position in the table.
That position is set by its proton number. The block name is the sub-shell that the outer electrons of the element fill last.
The Periodic Table shaded by block. An element is placed in the block that matches the sub-shell its outer electrons fill last.
| Block | Where it sits | Sub-shell filled last | Example |
|---|---|---|---|
| s block | Groups 1 and 2, plus H and He | s | Na (11) |
| p block | Groups 3 to 7 and Group 0, except He | p | Cl (17) |
| d block | The central block between Groups 2 and 3 | d | Fe (26) |
| f block | The two rows below the main table, proton numbers 58 to 71 and 90 to 103 | f | Nd (60), U (92) |
Exam focus: Use the position of the element to name its block, and state the reason: its position follows from its proton number.
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
You need to use data to illustrate periodic trends, including electronic configurations, atomic radii, first ionisation energies and melting or boiling points.
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 points
Determined by structure: metallic, giant covalent, or simple molecular/monatomic.
First ionisation energies
Generally increase across a period; decrease down a group. Small exceptions occur between Group 2 and Group 3, and between Group 5 and Group 6.
Quick Check: Matching the Argument to the Trend
Pick the one accurate statement in each round to finish the page.
Atomic Radius Across Period 3
AQA wants the trend in atomic radius stated and explained in its own right, not only as a step in the ionisation energy argument. From sodium to argon the atomic radius decreases.
| Element | Na | Mg | Al | Si | P | S | Cl |
|---|---|---|---|---|---|---|---|
| Atomic radius / nm | 0.191 | 0.160 | 0.143 | 0.118 | 0.110 | 0.104 | 0.099 |
| Nuclear charge | +11 | +12 | +13 | +14 | +15 | +16 | +17 |
Explanation. Across the period each element has one more proton in the nucleus, so the nuclear charge increases.
The added electron goes into the same shell (the third), so the shielding by the inner 1s, 2s and 2p electrons is almost the same for every element.
The outer electrons are therefore attracted more strongly and pulled closer to the nucleus.
Two data sets. The line graph higher up this page uses covalent radii, so sodium is 0.156 nm. This table and the bar chart use metallic radii for the three metals, so sodium is 0.191 nm. Both show the same fall.
Atomic radius across Period 3 from sodium to chlorine, falling as nuclear charge rises with constant shielding
Atomic radius falls across Period 3 as the nuclear charge rises while shielding stays almost constant.
Exam sentence: Atomic radius decreases across Period 3 because the nuclear charge increases while the outer electrons stay in the same shell with similar shielding, so they are attracted more strongly to the nucleus.
Quick Check: Spot the Flaw in the Argument
A student explains the fall in atomic radius the wrong way round. Find the error.
Common Exam Points
- Across a period, nuclear charge increases while shielding stays similar, so atomic radius decreases.
- Down a group, the extra shells and extra shielding make atomic radius increase and first ionisation energy decrease.
- Explain the melting point trend using structure and bonding, never ionisation energy.
- Silicon has the highest melting point in Period 3 because its giant covalent lattice has many strong covalent bonds to break.
- The melting point rises from sodium to aluminium because metallic bonding gets stronger: more delocalised electrons per atom and a smaller, more highly charged ion.
- Among the simple molecular elements, S8 has the most electrons, so it has the strongest van der Waals forces and the highest melting point.
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.
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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