Physical properties periodic trends
CIE A-Level ChemistryΒ· Unit 9: PeriodicityΒ· 15 min read
1. Trends in Atomic and Ionic Radiusβ β ββββ± 4 min
Atomic Radius
Half the distance between the nuclei of two adjacent bonded atoms of the same element
Example:
In a Clβ molecule, bond length is 198 pm, so Cl atomic radius is 99 pm
Across every period, atomic radius consistently decreases from left to right. This trend is explained by changes in nuclear charge and electron shielding, as outlined below:
Proton number increases across the period, so total positive nuclear charge increases
Outer electrons are added to the same main shell, so shielding from inner electrons remains almost constant
Increased net nuclear attraction pulls outer electrons closer, decreasing atomic radius
Explain why the atomic radius of sodium (Z=11) is larger than chlorine (Z=17)
- 1
Compare nuclear charge:
- 2
Sodium has 11 protons, chlorine has 17 protons, so chlorine has higher nuclear charge than sodium.
- 3
Compare electron shielding:
- 4
Both elements have outer electrons in the n=3 shell, so shielding is similar for both.
- 5
Conclusion:
- 6
Higher nuclear charge in chlorine creates a stronger attraction on outer electrons, pulling them closer, so chlorine has a smaller atomic radius than sodium.
For ionic radius, the trend splits into two parts: cations (left of the period) show decreasing radius across the period. Anions (right of the period) are all larger than cations of the same period (they have an extra electron shell), and also decrease in radius across the anions.
Exam tip:
Always explicitly distinguish between atomic and ionic radius trends in exam answers, they are not identical across a whole period.
2. Melting Point Trends Across Period 3β β β βββ± 5 min
Melting Point
The temperature at which a solid changes to liquid, dependent on the strength of attractions between particles in the solid structure.
Melting point does not change uniformly across Period 3: it increases from sodium to silicon, then drops sharply for non-metals from phosphorus to argon. This trend is entirely explained by changes in structure and bonding across the period.
Explain why melting point increases from Na to Al across Period 3
- 1
All three elements have a giant metallic structure, with positive metal ions held together by electrostatic attraction to delocalised electrons.
- 2
From Na to Al, the number of delocalised electrons per atom increases: 1 for Na, 2 for Mg, 3 for Al.
- 3
The charge on the metal ion also increases from +1 to +3, so electrostatic attraction between ions and delocalised electrons strengthens.
- 4
More energy is required to break stronger metallic bonds, so melting point increases from Na to Al.
Silicon has the highest melting point in Period 3 because it has a giant covalent network structure, with strong covalent bonds between all silicon atoms. Breaking these bonds requires a very large amount of energy.
From P to Ar, elements are simple molecular substances, with only weak London dispersion forces between discrete molecules. Little energy is needed to overcome these weak forces, so melting points are much lower than silicon.
3. Electrical Conductivity Trendβ β ββββ± 3 min
Electrical conductivity requires free, mobile charged particles that can carry current through a material. Across Period 3, conductivity follows a clear trend based on the presence of these free charged particles.
Explain why aluminium conducts electricity but solid sulfur does not
- 1
Aluminium has a giant metallic structure with delocalised free electrons that can move through the lattice and carry an electric current.
- 2
Sulfur exists as Sβ simple molecules, all electrons are held in covalent bonds, with no free mobile charged particles to carry current.
- 3
Hence aluminium is a good conductor, and sulfur does not conduct electricity.
Silicon is a special case: it is a semiconductor, with very low conductivity at room temperature that increases with increasing temperature. This sets it apart from metals (conductivity decreases with temperature) and non-metals (no conductivity at all).
Exam tip:
Remember that silicon is a semiconductor, not a full insulator: this is a common distinguishing point in exam questions.
4. Generalisation and Checkβ β β βββ± 3 min
The same general trends observed in Period 3 hold for all other periods: atomic radius decreases across the period, melting point follows a pattern of increasing up to the group 14 element (giant covalent) then dropping for simple molecular non-metals, and conductivity follows the same metal-semiconductor-insulator trend.
Test your understanding of core trends:
Which of the following correctly describes the trend in atomic radius across Period 3?
Increases from left to right
Decreases from left to right
Increases up to silicon then decreases
Decreases up to silicon then increases
Reveal answer
Decreases from left to right βCorrect! Atomic radius decreases across the whole period, as increasing nuclear charge with constant shielding pulls outer electrons closer.
Why does silicon have a higher melting point than sodium?
Silicon has a higher relative atomic mass
Silicon has stronger metallic bonding
Silicon has strong covalent bonds throughout its giant lattice
Silicon has more protons per atom
Reveal answer
Silicon has strong covalent bonds throughout its giant lattice βCorrect! Silicon is a giant covalent network, with much stronger bonding than sodium's metallic structure.
5. Common Pitfalls
Wrong move:
Stating that atomic radius increases across a period because more electrons are added
Why:
Extra electrons are added to the same shell, and increased nuclear charge has a larger effect than electron repulsion
Correct move:
State that atomic radius decreases across a period due to increasing nuclear charge with constant electron shielding
Wrong move:
Claiming ionic radius decreases continuously across the whole period
Why:
Anions have one extra electron shell compared to cations in the same period, so all anions are larger than cations
Correct move:
Split the trend: cations decrease in radius across the period, there is a step up, then anions also decrease in radius
Wrong move:
Saying melting point increases continuously across Period 3
Why:
After silicon, structure changes from giant to simple molecular, so melting point drops sharply
Correct move:
State melting point increases from Na to Si, then decreases sharply for non-metals from P to Ar
Wrong move:
Claiming silicon does not conduct electricity at all
Why:
Silicon is a semiconductor with low but non-zero conductivity, unlike the non-metals to its right
Correct move:
State conductivity is high for Na-Al, low for silicon (semiconductor), zero for P-Ar
Wrong move:
Explaining melting point trends only in terms of relative atomic mass
Why:
CIE requires answers to link melting point to bonding strength and structure, mass alone is not an explanation
Correct move:
Always link melting point to the strength of bonding holding the structure together, and state the type of structure
6. Quick Reference Cheatsheet
Property | Trend (left β right across Period 3) | Core Reason |
|---|---|---|
Atomic Radius | Decreases | Increasing nuclear charge, constant shielding |
Ionic Radius | Decreases (cations), jump up, decreases (anions) | Anions have extra electron shell |
Melting Point | Increases NaβSi, sharp decrease after | Metallic bonds strengthen β giant covalent β simple molecular |
Electrical Conductivity | High (Na-Al), low (Si), zero (P-Ar) | Delocalised electrons β semiconductor β no free charges |
7. Frequently Asked
Why does atomic radius decrease across a period?
Proton number (and thus nuclear charge) increases across the period, while outer electrons are added to the same shell so shielding stays similar. The stronger attraction from higher nuclear charge pulls outer electrons closer, reducing radius.
Why is melting point of silicon so much higher than phosphorus?
Silicon has a giant covalent network structure with strong covalent bonds across the entire lattice, requiring lots of energy to break. Phosphorus is simple molecular, with only weak London dispersion forces between P4 molecules, so little energy is needed to separate molecules.
When this came up on past exams
AI-estimated based on syllabus patterns β cross-check with official past papers for accuracy. Use only as revision-focus signals.
- 2023 Β· 2
Period 3 melting point trend explanation
- 2022 Β· 1
Atomic radius multiple choice
- 2021 Β· 2
Ionic radius trend comparison
What's Next
Understanding physical periodic trends is the foundation for explaining other key periodic trends in the CIE A-Level periodicity unit, including ionisation energy, chemical reactivity of elements, and acid-base properties of oxides. These trends are often combined with bonding and structure questions in both multiple choice and structured papers, so mastering the link between structure, bonding and physical properties here will help you access full marks on common explanation questions that appear almost every exam session.