GROUP 17 ELEMENTSGroup 17 elements – the halogens: F, Cl,
Br, I, At
The halogens have seven electrons in the outermost shell of their atoms (with the
general outer electronic structure ns^2 np^5 , where nis greater than one) and gain the
stable electronic configuration of a noble gas by either accepting an electron to form
a negatively charged ion, or by sharing an electron to form a covalent bond. Some of
their common properties are:
1.The non-metallic elements exist as stable diatomic molecules: F 2 , Cl 2 , Br 2 and I 2.
2.They are volatile elements: fluorine and chlorine are gases at room temperature,
whereas bromine and iodine are a liquid and volatile solid respectively. Solid
iodine sublimes when warmed.3.They are coloured; the colours of the elements are shown in Table 12.7.
4.Halogens combine directly with most metals to form ionic solids:
2Na(s)Cl 2 (g)2NaCl(s)4.Some iodides have covalent character, however, because of the high polarizability
of the iodide ion.
5.They form gaseous (with the exception of HF, bp 20 °C), covalent hydrides with
the general formula HX. The thermal stability of these hydrides decreases down
the group. The hydrides form acidic aqueous solutions.6.Their silver salts become increasingly covalent from AgF to AgI. A test for aque-
ous halides is the precipitation of the silver salt using silver nitrate solution. The
general ionic equation isAg(aq)X(aq)AgX(s)4.The halogen can be identified from the colour of the precipitate formed: AgCl is
white, AgBr is cream, and AgI is pale yellow. See p. 88.
- When exposed to light, silver chloride and bromide decompose into silver and
the halogen. The finely divided silver appears black. This is why silver halides are
used in photographic films.7.The reactivity of the halogens decreases as the group is descended. Chlorine will
displace bromine and iodine from bromides and iodides, respectively, and
bromine will displace iodine from iodides.Cl 2 (aq)2Br(aq)2Cl(aq)Br 2 (aq)4.In the reaction above, chlorine is acting as an oxidizing agent, the bromide ion is
acting as the reducing agent.
The trends apparent within the group are shown in Table 12.8.
12.5
203Table 12.7Colours of the halogens
Element Colour
Fluorine pale yellow gas
Chlorine pale green gas
Bromine red/brown liquid
Iodine black solid, purple in organic solvents or vapour phase