11.4 The p block: Group 7

Group 7

The halogens are in group 7. They have 7 electrons in the outer shell.
They are the most reactive group of non-metals, and do not occur naturally in the element form.
They are all found in compounds, often as halide ions (a single negatively charged ion, eg Br-). They all occur as diatomic molecules- atoms linked with a covalent bond
In compounds, a halogen atom gains stability by gaining an electron from a metal to form a halide ion (ionically bonded compound) or by sharing an electron from another atom (covalently bonded compound). In both cases, the halogen has a oxidation state of -1
Halogens (apart from fluorine) can expand the outer shell to hold more than 8 electrons

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The physical states moves from gas to liquid to solid as you go down group 7. This change is caused by an increase in the strength of intermolecular bonds of the diatomic molecule (instantaneous dipole- induced dipole attraction)
Fluorine is the most volatile halogen- this is because it has the smallest molecules, with the least number of electrons, so has the weakest intermolecular forces
The molecules get bigger as the group is descended -> more electrons per molecule -> stronger intermolecular forces -> change in physical state

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The halogens remove electrons from other elements- they are oxidising agents. The elements at the top of the group are the most reactive and are the strongest oxidising agents
Fluorine (at the top) has the smallest atoms, so the attraction between the nucleus and the extra electron is needed is strong, so the atom gains the e- more readily to become a negative ion
With many metals, the halogens react to form halide ions eg, KBr and CaCl2 (oxidation state -1)
With non-metals (and some p-block and transition metals), halogens form molecular compounds containing covalent bonds (oxidation state -1)
If two halide ions are mixed in solution, a displacement reaction takes place. For example-
Cl2 (aq) + 2K+I+ (aq) -> 2K+Cl- + I2 (aq)
These are also redox reactions- chlorine is reduced (0 to -1) and iodine is oxidised (-1 to 0). They are non-reversible, iodine will not liberate chlorine as it is less reactive
Oxidation of halides to halogen- 2X- -> X2 + 2e-
Silver halides are precipitated when a solution of silver ions is added to a solution containing Cl-, Br- or I-. These are precipitation reactions- Ag+ (aq) + X- (aq) -> AgX (s)

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