Seven outer electrons. Less reactive down the group.
Group 7 contains the halogens: non-metals with 7 outer electrons. Chlorine, bromine and iodine are familiar examples.
They exist as diatomic molecules: two atoms bonded together. Their formulae are Cl₂, Br₂ and I₂.
Iodine forms a purple vapour when heated. The solid itself is grey-black.
Down Group 7, reactivity decreases, but melting and boiling points increase as relative molecular mass increases.
Halogens react by gaining an electron, or by sharing electrons. Down the group, more shells mean greater distance and shielding. Attraction for an incoming electron is weaker, so gaining an electron becomes harder.
With metals, halogens form ionic compounds called metal halides. The halogen gains one electron to form a −1 ion.
With non-metals, halogens form covalent compounds, sharing electrons. For example, hydrogen reacts with chlorine to make hydrogen chloride, HCl.
A more reactive halogen displaces a less reactive halogen from a solution of its salt.
Chlorine > bromine > iodine in reactivity. Chlorine displaces bromine and iodine; bromine displaces iodine. The reverse reactions do not happen.
Seven outer electrons; diatomic molecules; reactivity decreases; boiling points increase; more reactive halogens displace less reactive ones.
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