The Halogens are 5 non metallic group 7 elements in the periodic table. They have slightly different properties because of their electronic setting (having 7 electrons in their outermost shells).
In terms of patterns and trends, as you move further down the group, electronegativity and reactivity decreases, while boiling points increase. (Exception of Astatine, because that stuff's radioactive).
Halogens have a high electronegativity, and are particularly reactive to form stable ionic crystals (Salts) as the name suggests, halo meaning salt and gen meaning to form.
**Sodium Chloride is the most abundant Halogen-produced Salt**
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| Sodium Chloride Crystals |
The charge density of the nucleus of Halogens is important because it shows that they have a high proton charge, which grants them a good electronegativity making them ideal anions, for pairing with the 1st group cations such as Na+.
Halogens, as they have 7 electrons in their outer shells, have an oxidation number of -1, with a few exceptions, for example if it was binded with Oxygen or another Halogen that is more reactive, with Fluorine being the most reactive one.
The difference in reactivity between the group 7 elements is within the fact that they're in consecutive periods, meaning there's a hierarchy relating between the number of outermost shells and the reactivity of the element. This is relating to the idea that more rings around the nucleus make the outer-most shell further away from the centre point of electronegativity, so the pull is spread over a large area decreasing its concentration.
Halogens as oxidation agents follow this general form:
