Characteristic physical properties
(a) existence of halogens as diatomic molecules and explanation of the trend in the boiling points of Cl 2, Br2 and I2, in terms of induced dipole–dipole interactions (London forces) (see also 2.2.2 k)
Redox reactions and reactivity of halogens and their compounds
(b) the outer shell s2p5 electron configuration and the gaining of one electron in many redox reactions to form 1– ions
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(c) the trend in reactivity of the halogens Cl 2, Br2 and I2, illustrated by reaction with other halide ions
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(d) explanation of the trend in reactivity shown in (c), from the decreasing ease of forming 1– ions, in terms of attraction, atomic radius and electron shielding
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(e) explanation of the term disproportionation as oxidation and reduction of the same element, illustrated by:
(i) the reaction of chlorine with water as used in water treatment
(ii) the reaction of chlorine with cold, dilute aqueous sodium hydroxide, as used to form bleach
(ii) reactions analogous to those specified in (i) and (ii)
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(f) the benefits of chlorine use in water treatment (killing bacteria) contrasted with associated risks (e.g. hazards of toxic chlorine gas and possible risks from formation of chlorinated hydrocarbons)
Characteristic reactions of halide ions
(g) the precipitation reactions, including ionic equations, of the aqueous anions Cl –, Br– and I– with aqueous silver ions, followed by aqueous ammonia, and their use as a test for different halide ions.
