Explain how properties of the elements in Group 7 depend on the outer…

The idea

Group 7 elements are the halogens: fluorine, chlorine, bromine, iodine and astatine. Each atom has seven electrons in its outer shell, giving it a configuration that is one electron short of a stable noble-gas arrangement. This single feature explains most of their chemistry. When a halogen atom gains one electron it forms a 1⁻ ion with a full outer shell, so halogens are reactive non-metals and strong oxidising agents. Two halogen atoms can also share a pair of electrons in a single covalent bond, forming diatomic molecules such as Cl₂. Because the outer shell is one electron short, the halogens react vigorously with metals such as sodium, forming salts like sodium chloride. Reactivity decreases down the group because the outer shell is further from the nucleus and shielded by more inner shells, so the attraction for an incoming electron is weaker.

What earns the marks

• State that a Group 7 atom has seven electrons in its outer shell.
• Explain that gaining one electron gives a full outer shell and forms a 1⁻ ion, for example Cl + e⁻ → Cl⁻.
• Link the tendency to gain an electron to reactivity as non-metals and oxidising agents.
• Explain that two atoms can share electrons to form diatomic molecules such as Cl₂.
• Describe the trend in reactivity down the group in terms of increased distance and shielding of the outer shell from the nucleus.
• Use a named example, such as chlorine reacting with sodium to form sodium chloride, to support the explanation.

Common mistakes and how to put them right

• Saying halogens lose seven electrons to gain a full outer shell.

  Put it right by stating that gaining one electron is far more favourable and forms a 1⁻ ion.

• Writing that reactivity increases down Group 7.

  Put it right by explaining that the outer shell is further from the nucleus and more shielded, so the attraction for an incoming electron is weaker.

• Confusing the outer-shell electron count with the group number for other groups.

  Put it right by stating that for Group 7 the group number equals the number of outer-shell electrons, which is seven.

Exam tips

• Always link the outer-shell electron count to a specific consequence, such as ion formation, covalent bonding or reactivity, rather than just stating the number seven.
• When explaining the reactivity trend, refer explicitly to distance from the nucleus and shielding by inner electron shells.
• Use correct notation for ions and molecules, for example Cl⁻ and Cl₂, and balance any equation you write.