A-level Chemistry/AQA/Module 2/Group VII: The Halogens
Physical Properties 
The elements contained in group VII of the periodic table are often referred to as the Halogens, or Halide ions when they exist in their anion (negatively charged) form e.g. the halide of fluorine is F-. There are generally four Halogens that we are concerned with; Fluorine (F2), Chlorine (Cl2), Bromine (Br2) and Iodine (I2).
At room temperature, these 4 elements exist as the following:
Fluorine: Yellow gas
Chlorine: Yellow-Green gas
Bromine: Red-Brown liquid
Iodine: Dark purple solid - sublimes to a purple vapour
The halogens are highly reactive elements and exist covalently bonded in a diatomic state (X2). Due to their high reactivity, they never occur in the free state, but instead are always found combined with other elements, forming compounds.
Size of atoms 
The atoms get bigger as you go down the group because each element has one extra filled main level of electrons compared with the one above it.
In the case of fluorine existing as a diatomic molecule the F-F bond is unexpectedly weak. This is due to the tiny size of these atoms being the smallest of the halogens and the electrons when bonded being so close together leads to a repulsion occurring between electrons that are not bonded.
Boiling points 
The boiling points of halogens increase down the group due to their increasing strength of the Van der Waals forces as the size and relative atomic mass of the atoms increase. This change manifests itself in the change in physical state of the of the elements down the group from gaseous fluorine to solid iodine.
This is the ability of an atom to attract electrons or electron density towards itself within a covalent bond. Electronegativity depends upon the attraction between the nucleus and bonding electrons in the outer shell which inturn depends on the balance between the number of protons in the nucleus (nuclear charge), the distance between the nucleus and bonding electrons and the shielding effect of inner electrons.
Hydrogen halides (HX) where X is the halogen the H-X bond gets further away from the nucleus as the atoms get larger going down the group. This makes the shared electrons further from the halogen nucleus and increases the shielding by more inner electrons thus electronegativity decreases down the group.
Displacement reactions 
Halogens will react with metal halides in solution in such a way that the halide will be displaced by a more reactive one, these reactions are known as displacement reactions such as the following.
Cl2 + 2NaBr → Br2 + 2NaCl
2Na+ + 2Br- → Br2 + 2Na+ + 2Cl-
The two sodium ions are spectator ions.
In this reodx reaction chlorine is acting as an oxidising agent by the removing electrons from Br- and so oxidising 2Br- to Br2. In general a halogen will displace the ion of a halogen below it in the periodic table.