AQA · A-Level · Chemistry
Course version: 7405 · Version from Sept 2016
The trends in atomic radius, first ionisation energy and melting point of…
Full specification wording
Physical properties of Period 3 elements: The trends in atomic radius, first ionisation energy and melting point of the elements Na–Ar The reasons for these trends in terms of the structure of and bonding in the elements.
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.
Checked against the AQA 7405 specification (opens in a new tab) on .
- Point 1 of 2
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
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. 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.
What earns the marks
- Atomic radius decreasing across the period as nuclear charge increases with similar shielding.
- First ionisation energy increasing overall across the period.
- Explaining the dip at aluminium by the electron coming from a 3p sub-shell that is higher in energy and shielded by 3s.
- Explaining the dip at sulfur by repulsion between the two electrons paired in one 3p orbital.
- The melting point sequence, with the giant metallic elements rising, silicon highest as giant covalent, then the simple molecular elements, and finally monatomic argon.
Common mistakes and how to put them right
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.
Exam 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.
Try the lesson
The start of the Physical properties of Period 3 elements: The trends in atomic radius, first ionisation energy and melting point of the elements Na–Ar The reasons for these trends in terms of the structure of and bonding in the elements. lesson, made from checked teaching notes for this specification point.
Have a go first
Which of the following describes the change in atomic radius across Period 3 from sodium to argon?
Listen
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. 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. A strong answer earns marks for these points. atomic radius decreasing across the period as nuclear charge increases with similar shielding. first ionisation energy increasing overall across the period. explaining the dip at aluminium by the electron coming from a 3p sub-shell that is higher in energy and shielded by 3s. explaining the dip at sulfur by repulsion between the two electrons paired in one 3p orbital. Exam tip: Learn the two dips by their cause, not their position; the cause is what the mark scheme credits.
Key words
Say what each word means before you turn the card.
atomic radius
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first ionisation energy dips
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giant, simple molecular and monatomic structures
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melting point and bonding
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See every definition
- atomic radius
- Falls steadily across Na to Ar because the outer electrons are pulled in harder as nuclear charge rises with similar shielding.
- first ionisation energy dips
- Rises overall across the period but dips twice, at aluminium and at sulfur, for reasons of sub-shell energy and electron repulsion.
- giant, simple molecular and monatomic structures
- Melting point follows structure: giant metallic and giant covalent elements, then simple molecular elements, then monatomic argon.
- melting point and bonding
- Follows structure rather than radius, with weak van der Waals forces between simple molecular and monatomic elements.
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