So group seven, aka the halogens. Elements in group seven have a number of similar properties, most importantly they have low melting and boiling points. This is of course a typical property of non-metals.

From the lowest boiling and melting point to the highest, the group in order is fluorine, chlorine, bromine, iodine and astatine.

As you move down the group the halogens become darker in colour. For example fluorine is a very pale yellow whereas iodine will be dark purple in colour when it is in a vapour state.

Each element has five electrons in the outer p shell. For each element down the group, we add an extra shell. The result is that each successive element has a larger atomic radii than the one before.

Fluorine has the highest effective nuclear charge, as the outer electrons have little shielding from inner electrons. As a result it has the highest electronegativity. For each extra shell, the shielding increases, and the elements have lower electronegativity.