Physical properties of Period 3 elements — AQA A-Level Chemistry
Test yourself on Physical properties of Period 3 elements with AQA A-Level practice questions.
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Physical properties of Period 3 elements explained
Across Na to Ar the nuclear charge rises by one proton per element while the outer electrons all enter the same third shell, so shielding hardly changes.
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The outer electrons are therefore pulled in harder: atomic radius falls steadily and first ionisation energy rises overall, dipping twice. It dips at aluminium, whose outer electron is in a 3p sub-shell that is higher in energy and shielded by the 3s pair, and at sulfur, where two electrons paired in one 3p orbital repel each other. Melting point follows structure rather than radius. Na, Mg and Al are giant metallic and rise in turn, as the ions become smaller and more highly charged and release more delocalised electrons. Silicon is giant covalent and highest of all. Then P₄, S₈ and Cl₂ are simple molecular, while Ar is monatomic. They have weak van der Waals forces, so S₈ melts above P₄, above Cl₂, above Ar.
Students should be able to: explain the trends in atomic radius and first ionisation energy explain the melting point of the elements in terms of their structure and bonding.
Marks come from naming mechanisms, not describing graphs. The atomic radius trend must be explained: it decreases across Period 3 because nuclear charge increases while shielding remains similar, pulling the outer shell closer. An ionisation energy comparison needs three controlling factors: nuclear charge, distance of the outer electron from the nucleus, and shielding by inner shells, and must compare two elements. A melting point answer needs the structure named (giant metallic, giant covalent, simple molecular or monatomic) and the force overcome named alongside it. Aluminium melts above sodium because Al³⁺ is smaller, more highly charged and contributes three delocalised electrons, making the attraction to the sea of electrons stronger. Sulfur melts above phosphorus because S₈ has more electrons than P₄, so van der Waals forces are stronger.
Your focus
- Account for the fall in first ionisation energy between magnesium and aluminium in terms of sub-shells.
- Rank sodium, silicon and sulfur by melting point and justify each position from its structure and bonding.
- Predict which element from sodium to argon has the lowest melting point and explain the answer using van der Waals forces between monatomic argon atoms.
Show all 6 objectives
- Explain why atomic radius decreases across Period 3 in terms of nuclear charge and shielding.
- Write a three-factor explanation for why the first ionisation energy of magnesium exceeds that of sodium.
- Explain why aluminium melts at a higher temperature than sodium, referring to ionic charge, ionic size and delocalised electrons.
Physical properties of Period 3 elements exam tips
Marking Points
- one mark for atomic radius decreasing across the period as nuclear charge increases with similar shielding
- one mark for first ionisation energy increasing overall across the period
- one mark for explaining the dip at aluminium by the electron coming from a 3p sub-shell that is higher in energy and shielded by 3s
- one mark for explaining the dip at sulfur by repulsion between the two electrons paired in one 3p orbital
- one mark for the melting point sequence, with the giant metallic elements rising, silicon highest as giant covalent, then the simple molecular elements, and finally monatomic argon
- one mark for explaining that atomic radius decreases across the period due to increasing nuclear charge and similar shielding
- one mark for naming all three factors controlling ionisation energy: nuclear charge, distance and shielding
- one mark for naming the structure as giant metallic, giant covalent, simple molecular or monatomic
- one mark for naming the force overcome on melting, such as the attraction between metal ions and delocalised electrons
- one mark for making a comparison between two named elements rather than describing one in isolation
Examiner Tips
- 💡Learn the two dips by their cause, not their position; the cause is what the mark scheme credits.
- 💡For any melting point comparison, name the structure first and the force overcome second.
- 💡Use a fixed three-part sentence for ionisation energy answers so you never drop one of the factors.
- 💡The word 'stronger' must always be followed by the name of the force that is stronger.
Common Mistakes
- claiming sodium has the lowest melting point of the period, when Cl₂ melts at 172 K and sodium at 371 K
- explaining the melting of silicon by breaking van der Waals forces rather than strong covalent bonds
- placing the ionisation energy dips anywhere other than at aluminium and sulfur
- saying covalent bonds are broken when sulfur melts
- claiming that argon is simple molecular rather than monatomic
- omitting the atomic radius trend or failing to mention that shielding remains similar across the period
- writing that an atom 'wants' to lose an electron rather than giving an explanation based on attraction
- saying intermolecular forces are broken when a giant covalent lattice melts
- writing 'the atom gets bigger' without saying which of the three factors has changed
- describing the melting of magnesium as breaking bonds between atoms rather than between Mg²⁺ ions and delocalised electrons