The elements in a group of the Periodic Table have similar properties. Group 1, the alkali metals (lithium, sodium and potassium), are soft metals with low melting points and low densities. Lithium, sodium and potassium all float on water. They react with water to make a metal hydroxide and hydrogen gas, and the solution that forms is alkaline. They get more reactive as you go down the group.
Group 7, the halogens, are non-metals made of diatomic molecules such as Cl2. Chlorine is a pale green gas, bromine is an orange-brown liquid and iodine is a grey solid. Melting points and boiling points increase down the group, but reactivity decreases. Because of this, a more reactive halogen will displace a less reactive halogen from a solution of its halide. Chlorine added to potassium bromide solution makes potassium chloride and orange bromine.
Group 0, the noble gases (helium, neon and argon), are colourless gases made of single atoms. They are unreactive, and their boiling points increase down the group.