The alkali metals in Group 1 (lithium, sodium and potassium) are soft enough to cut with a knife and are shiny when freshly cut. They have low melting points and a low density, so lithium, sodium and potassium all float on water. They react with water to make a metal hydroxide and hydrogen gas, and the solution formed is alkaline. For example, 2Na + 2H2O → 2NaOH + H2. Reactivity increases down the group, so potassium reacts more vigorously than sodium.
The halogens in Group 7 are non-metals that exist as diatomic molecules such as Cl2 and Br2. 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. A more reactive halogen displaces a less reactive halogen from a solution of its halide. When chlorine water is added to potassium bromide solution, chlorine displaces bromine and the solution turns orange: Cl2 + 2KBr → 2KCl + Br2. Chlorine also displaces iodine from potassium iodide, giving a brown solution.
The noble gases in Group 0 (helium, neon and argon) are colourless gases. They exist as single atoms, so they are monatomic. They are unreactive, and their boiling points increase down the group.