Describe Period 3 physical-property trends
Read qualitative changes in atomic radius, ionic radius, melting point and electrical conductivity across Period 3.
Section 9 · The Periodic Table: chemical periodicity
AS Level; Cambridge International Chemistry 9701, 2025–2027 syllabus. Period 3 trends are qualitative unless supplied data are given. Equations show the stated products and conditions; observations and pH depend on concentration and experimental conditions. Written practice is self-checked
Before you begin
- Use electron configurations, oxidation numbers, balanced equations and ionic charges.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What is the general atomic-radius trend across Na to Cl?
What you will learn
- Describe atomic-radius variation across Period 3
- Compare the Period 3 cation and anion radius sequences
- Describe melting-point and electrical-conductivity patterns
Atomic and ionic radii do not follow one interchangeable sequence
Across Na to Cl, atomic radius generally decreases: proton number rises while the added electrons enter the same principal shell, so the stronger nuclear attraction pulls the outer electron cloud closer. Argon is commonly omitted from covalent-radius comparisons because it does not form ordinary covalent bonds; use the radius definition supplied.
For Period 3 ions, Na⁺, Mg²⁺ and Al³⁺ are isoelectronic with 10 electrons. Their radii decrease as nuclear charge rises. P³⁻, S²⁻ and Cl⁻ are isoelectronic with 18 electrons; their radii also decrease across that sequence. The anions are much larger than the cations because the anion sequence has an extra occupied electron shell.
Melting point and conductivity show structural changes
Melting points rise from Na through Mg and Al to a maximum at giant-covalent Si. They then fall sharply for molecular P₄, rise somewhat for larger S₈ molecules, and are low for Cl₂. The pattern is not a smooth periodic increase or decrease; bonding and particle structure change across the period.
Na, Mg and Al conduct electricity as metals with mobile, delocalised electrons. Silicon is a semiconductor. Solid phosphorus and sulfur and molecular chlorine do not conduct in the usual model because they lack mobile charge carriers. State the phase and conditions when using conductivity as evidence.
Pause and explain
Why is Al³⁺ smaller than Na⁺ in the Period 3 isoelectronic cation sequence?
Worked example
Describe the overall atomic-radius trend from Na to Cl and the melting-point pattern from Na to Si.
Show the worked solution
- From Na to Cl, atomic radius generally decreases across the same period.
- Melting points rise from Na through Mg and Al to Si.
- The different patterns reflect increasing nuclear attraction for radius and changing structures/bonding for melting point.
Answer Atomic radius generally decreases from Na to Cl; melting point rises from Na to a maximum at Si as bonding changes from metallic to giant covalent.
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Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Put Na⁺, Mg²⁺ and Al³⁺ in order of increasing ionic radius and explain the order.
Give me a hint
They have the same number of electrons; compare proton number.
Compare my reasoning
- Each ion has 10 electrons.
- Nuclear charge rises from Na⁺ to Mg²⁺ to Al³⁺.
- Greater attraction makes the radius smaller.
Al³⁺ < Mg²⁺ < Na⁺ in radius; the isoelectronic ions have increasing nuclear charge in that order.
Look for these in your work
- Isoelectronic sequence recognised
- Order correct
- Nuclear attraction explained
State the general changes in atomic radius and melting point from Na to Si.
Give me a hint
Separate the electron-cloud trend from the structure-dependent melting trend.
Compare my reasoning
- Atomic radius generally decreases across the period.
- Melting point rises from Na through Mg and Al to Si.
- The melting point reaches its Period 3 maximum at giant-covalent Si.
Atomic radius generally decreases; melting point rises to a maximum at Si.
Look for these in your work
- Both trends stated
- Silicon maximum identified
- Different reasons kept distinct
A table lists Na, Mg, Al, Si, P, S and Cl. Describe the broad conductivity pattern and explain the change after Al.
Give me a hint
Relate the first three metals to their charge carriers, then identify the structures to the right.
Compare my reasoning
- Na, Mg and Al conduct as metals with mobile electrons.
- Si is a semiconductor; the non-metals to its right are poor conductors in the usual states.
- The bonding and available mobile charge carriers change across the period.
Conductivity is strong for Na–Al, lower for semiconducting Si and poor for P, S and Cl because the structures and mobile charge carriers change.
Look for these in your work
- Metallic region identified
- Semiconductor distinguished
- Bonding/carriers linked
Common mistakes
- Saying every Period 3 property changes smoothly
- Mixing cation and anion radii into one decreasing sequence
- Including Ar in a covalent-radius sequence without checking the definition
- Calling silicon a typical metal or a simple molecular solid
What can you explain now?
What is the general atomic-radius trend across Na to Cl?
Compare with the explanation
It generally decreases
Increasing nuclear charge attracts electrons in the same main shell more strongly.
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Make it stick
Which Period 3 patterns are smooth, which show a maximum or discontinuity, and what structural evidence helps explain them?
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