These elements are found in the same group as they have the same number of electrons in their outer shells.
They have similar properties as they all need one electron to get a full outer shell.
Atomic Size – Increases down the group, as the atoms have more shells
Density – Increases down the group, as the atomic mass increases (more protons and neutrons).
Reactivity – Decreases down the group, as it is harder for the nucleus to attract an outer electron, as the outer shell is further from the nucleus and the inner shells shield the attractive force of the nucleus.
It is important to know the reactivity of the halogens with iron wool and with halide salt solutions in displacement reactions: