AQA · A-Level · Chemistry

    Course version: 7405 · Version from Sept 2016

    Students should be able to: explain the trends in atomic radius and first…

    Full specification wording

    Physical properties of Period 3 elements: 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.

    Checked against the AQA 7405 specification (opens in a new tab) on .

    • Point 2 of 2
    • 4 key terms

    Study notes

    From the checked teaching notes for this specification point.

    The idea

    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.

    What earns the marks

    • Explaining that atomic radius decreases across the period due to increasing nuclear charge and similar shielding.
    • Naming all three factors controlling ionisation energy: nuclear charge, distance and shielding.
    • Naming the structure as giant metallic, giant covalent, simple molecular or monatomic.
    • Naming the force overcome on melting, such as the attraction between metal ions and delocalised electrons.
    • Making a comparison between two named elements rather than describing one in isolation.

    Common mistakes and how to put them right

    • 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.

    Exam tips

    • 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.

    Try the lesson

    The start of the Physical properties of Period 3 elements: 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. lesson, made from checked teaching notes for this specification point.

    Have a go first

    Which element in Period 3 has the highest melting point?

    Listen

    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. A strong answer earns marks for these points. explaining that atomic radius decreases across the period due to increasing nuclear charge and similar shielding. naming all three factors controlling ionisation energy: nuclear charge, distance and shielding. naming the structure as giant metallic, giant covalent, simple molecular or monatomic. naming the force overcome on melting, such as the attraction between metal ions and delocalised electrons. Exam tip: Use a fixed three-part sentence for ionisation energy answers so you never drop one of the factors.

    See it

    A top-down flowchart. The first node, Period 3 element, leads to a node labelled Name the structure. This branches into four structure types: Giant metallic, Giant covalent, Simple molecular and Monatomic. All four lead to a single node labelled Name the force overcome on melting, which leads to a final node labelled Compare two named elements.
    It shows how a melting point answer is built: name the structure, name the force overcome, then compare two named elements.

    Key words

    Say what each word means before you turn the card.

    atomic radius

    Try answering aloud or in your head before turning the card.

    three ionisation energy factors

    Try answering aloud or in your head before turning the card.

    naming structures

    Try answering aloud or in your head before turning the card.

    metallic bonding strength

    Try answering aloud or in your head before turning the card.

    See every definition
    atomic radius
    Decreases across Period 3 because nuclear charge increases while shielding remains similar, pulling the outer shell closer.
    three ionisation energy factors
    Nuclear charge, distance of the outer electron from the nucleus, and shielding by inner shells, used to compare two elements.
    naming structures
    The structure must be named as giant metallic, giant covalent, simple molecular or monatomic, with the force overcome named alongside it.
    metallic bonding strength
    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.

    Label the diagram

    Some boxes have lost their names. Pick the right name for each number.

    The diagram again, with 3 boxes numbered instead of named. A top-down flowchart. The first node, Period 3 element, leads to a node labelled Name the structure. This branches into four structure types: Giant metallic, Giant covalent, Simple molecular and Monatomic. All four lead to a single node labelled Name the force overcome on melting, which leads to a final node labelled Compare two named elements.

    • 1Box 1

    • 2Box 2

    • 3Box 3

    0 of 3 labelled

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