Topic 4B - Elements of Group 7 (Halogens)

• The Group 7 elements are highly reactive non-metals. In order to achieve a full outer shell, halogens gain an electron to form a 1- ion.
• Atomic radius of Group 7 elements increases down the group due to the addition of electron shells.
• Electronegativity decreases down Group 7 due to increased atomic radius and electron shielding, meaning that electrons in the outer shells are less strongly attracted to the nucleus, and are more easily removed.
• Group 7 elements are simple covalent molecules, held together by London forces. The strength of these intermolecular forces increases down the group due to increased Ar. Therefore, more energy is required to overcome these intermolecular forces.
• Group 7 elements must gain an electron in order to react. As atomic radius increases down the group, this becomes harder since the nuclear charge is weakened by electron shielding. Down Group 7 reactivity therefore decreases.
• The halogens act as good oxidising agents as they accept electrons from the species being oxidised and are reduced. This oxidising power decreases down the goroup as their ability to attract electrons decreases due to shielding and greater atomic radius.
• The relative oxidising strengths mean a halogen will displace any halide beneath it in the Periodic Table. E.g. Cl2 will displace Br- and I- ions.
• The negative ions of halogens are known as halide ions. These ions are good reducing agents as they donate electrons to the species being reduced, becoming oxidised themselves. This reducing power increases down the group as electrons are more easily lost from larger ions due to shielding and a larger atomic radius.
• The redox reactions of Group 7 halides and H2SO4 vary depending on the reducing ability of the halide. The reducing ability of the halide can be observed by the relative reactions of the halides with sulfuric acid.
• NaF + H2SO4 -> NaHSO4 + HF

  NaCl + H2SO4 -> NaHSO4 + HCl

  No further reactions will occur since HF and HCl are not strong enough reducing agents to reduce H2SO4. HF and HCl will both be observed as misty fumes.
• NaBr + H2SO4 -> NaHSO4 + HBr

  2HBr + H2SO4 -> Br2 + SO2 + 2H2O

  HBr will be observed as misty fumes in the first reaction. Orange fumes of Br2 and choking fumes of SO2 will be observed in the second reaction.