AQA · A-Level · Chemistry
Course version: 7405 · Version from Sept 2016
The trends in electronegativity and boiling point of the halogens.
Full specification wording
Trends in properties: The trends in electronegativity and boiling point of the halogens.
Two trends that run in opposite directions.
Checked against the AQA 7405 specification (opens in a new tab) on .
- Point 1 of 7
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Two trends that run in opposite directions. Electronegativity falls from fluorine at 4.0 to iodine at 2.5: going down the group the atom has more shells, so the bonding pair of electrons lies further from the nucleus and is screened by more inner electrons, and the attraction for it weakens despite the larger nuclear charge. Boiling point rises: F₂ and Cl₂ are gases, Br₂ is a red-brown liquid and I₂ is a grey-black solid that sublimes to a purple vapour. The halogens are simple molecular, so boiling only separates molecules and the covalent bond inside each molecule is untouched. Each step down the group adds electrons, so the electron cloud is larger and more easily distorted, the induced dipoles are bigger, and the van der Waals forces between molecules take more energy to overcome.
What earns the marks
- Electronegativity decreasing down the group.
- Explaining that by increasing atomic radius and shielding, so the nucleus attracts the shared pair less strongly.
- Boiling point increasing down the group.
- Stating that the halogens are simple molecular and that van der Waals forces are overcome on boiling.
- Linking stronger van der Waals forces to more electrons and a more polarisable electron cloud.
Common mistakes and how to put them right
Saying the covalent bond within the molecule is broken when a halogen boils.
Explaining the boiling point trend by relative molecular mass alone, with no mention of electrons or polarisability.
Claiming electronegativity increases down the group because nuclear charge increases.
Calling van der Waals forces 'bonds'.
Describing iodine vapour as black rather than purple.
Exam tips
- Always say what is overcome on boiling, because 'the forces get stronger' with no name attached scores nothing.
- Learn the colours and states of chlorine, bromine and iodine as a set; they are asked for directly.
Try the lesson
The start of the Trends in properties: The trends in electronegativity and boiling point of the halogens. lesson, made from checked teaching notes for this specification point.
Have a go first
What happens to the boiling point of the halogens as you go down Group 7?
Listen
Two trends that run in opposite directions. Electronegativity falls from fluorine at 4.0 to iodine at 2.5: going down the group the atom has more shells, so the bonding pair of electrons lies further from the nucleus and is screened by more inner electrons, and the attraction for it weakens despite the larger nuclear charge. Boiling point rises: F₂ and Cl₂ are gases, Br₂ is a red-brown liquid and I₂ is a grey-black solid that sublimes to a purple vapour. The halogens are simple molecular, so boiling only separates molecules and the covalent bond inside each molecule is untouched. Each step down the group adds electrons, so the electron cloud is larger and more easily distorted, the induced dipoles are bigger, and the van der Waals forces between molecules take more energy to overcome. A strong answer earns marks for these points. electronegativity decreasing down the group. explaining that by increasing atomic radius and shielding, so the nucleus attracts the shared pair less strongly. boiling point increasing down the group. stating that the halogens are simple molecular and that van der Waals forces are overcome on boiling. Exam tip: Always say what is overcome on boiling, because 'the forces get stronger' with no name attached scores nothing.
See it
Key words
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electronegativity down a group
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van der Waals forces
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simple molecular structure
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colours and states
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See every definition
- electronegativity down a group
- Falls from fluorine at 4.0 to iodine at 2.5, because the atom has more shells so the bonding pair lies further from the nucleus and is screened by more inner electrons.
- van der Waals forces
- Forces between molecules that take more energy to overcome as each step down the group adds electrons, making the electron cloud larger and more easily distorted and the induced dipoles bigger.
- simple molecular structure
- The halogens are simple molecular, so boiling only separates molecules and the covalent bond inside each molecule is untouched.
- colours and states
- F₂ and Cl₂ are gases, Br₂ is a red-brown liquid and I₂ is a grey-black solid that sublimes to a purple vapour.
Label the diagram
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3Box 3
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