AQA · GCSE · Chemistry
Course version: 8462 · Version from Sept 2018
The elements in Group 7 of the periodic table are known as the halogens…
Full specification wording
Group 7: The elements in Group 7 of the periodic table are known as the halogens and have similar reactions because they all have seven electrons in their outer shell.
Group 7 contains fluorine, chlorine, bromine, iodine and astatine, collectively called the halogens.
Checked against the AQA 8462 specification (opens in a new tab) on .
- Point 1 of 7
- 2 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Group 7 contains fluorine, chlorine, bromine, iodine and astatine, collectively called the halogens. Each halogen atom has seven electrons in its outer shell, one short of a full shell. This shared outer-shell feature explains why halogens show similar reactions: they gain one electron to form a 1⁻ ion, or share one electron pair in a covalent bond, so completing the outer shell. Similarity of reactions does not mean identical reactivity. Down the group, atomic radius increases and shielding increases, so the nucleus attracts an incoming electron less strongly. Reactivity therefore decreases down Group 7: fluorine is the most reactive halogen and iodine is much less reactive. A displacement reaction, such as chlorine displacing bromine from potassium bromide solution, provides evidence for this trend.
What earns the marks
- Identify the halogens as the elements in Group 7 of the periodic table.
- State that each halogen atom has seven electrons in its outer shell.
- Explain that halogens gain one electron to form a 1⁻ ion or share one electron pair in a covalent bond.
- Link the common outer-shell configuration to the similar reactions of the halogens.
- Explain that reactivity decreases down Group 7 because the outer shell is further from the nucleus and more shielded.
- Use a displacement reaction as evidence that a more reactive halogen can displace a less reactive one from its compound.
Common mistakes and how to put them right
Saying halogens have seven electrons in total rather than seven in the outer shell.
Put it right by specifying the outer shell.
Writing halide ions with a baseline numeral and postfix sign.
Put it right by using superscript forms such as Cl⁻ and Br⁻.
Claiming all halogens react identically.
Put it right by saying reactions are similar in type but reactivity decreases down the group.
Confusing halogens with halide ions.
Put it right by noting that halogens are the elements, while halides are the 1⁻ ions formed from them.
Exam tips
- Begin similarity answers with the outer-shell electron count, because that is the cause of the shared chemistry.
- When explaining reactivity trends, compare distance from the nucleus and shielding between the two named halogens.
- For displacement questions, name the more reactive halogen and state which ion it replaces.
- Use superscript charges for all ions, for example F⁻, Cl⁻, Br⁻ and I⁻.
Try the lesson
The start of the Group 7: The elements in Group 7 of the periodic table are known as the halogens and have similar reactions because they all have seven electrons in their outer shell. lesson, made from checked teaching notes for this specification point.
Have a go first
What name is given to the elements in Group 7 of the periodic table?
Listen
Group 7 contains fluorine, chlorine, bromine, iodine and astatine, collectively called the halogens. Each halogen atom has seven electrons in its outer shell, one short of a full shell. This shared outer-shell feature explains why halogens show similar reactions: they gain one electron to form a 1⁻ ion, or share one electron pair in a covalent bond, so completing the outer shell. Similarity of reactions does not mean identical reactivity. Down the group, atomic radius increases and shielding increases, so the nucleus attracts an incoming electron less strongly. Reactivity therefore decreases down Group 7: fluorine is the most reactive halogen and iodine is much less reactive. A displacement reaction, such as chlorine displacing bromine from potassium bromide solution, provides evidence for this trend. A strong answer earns marks for these points. Identify the halogens as the elements in Group 7 of the periodic table. State that each halogen atom has seven electrons in its outer shell. Explain that halogens gain one electron to form a 1⁻ ion or share one electron pair in a covalent bond. Link the common outer-shell configuration to the similar reactions of the halogens. A common slip: Saying halogens have seven electrons in total rather than seven in the outer shell. Instead, specifying the outer shell. Exam tip: Begin similarity answers with the outer-shell electron count, because that is the cause of the shared chemistry.
Key words
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Halogens
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Trends in reactivity down Group 7
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See every definition
- Halogens
- The elements in Group 7 of the periodic table, which have similar reactions because they all have seven electrons in their outer shell.
- Trends in reactivity down Group 7
- Reactivity decreases down Group 7 because atomic radius increases and shielding increases, so the nucleus attracts an incoming electron less strongly.
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