Groups 1, 7 and 0 · Year 9
Three groups of the periodic table show clear patterns. The alkali metals (group 1) are soft, reactive metals that get more reactive going down. The halogens (group 7, or group 17 in the 1–18 numbering) are non-metals that exist as two-atom molecules and get less reactive going down. The noble gases (group 0, or group 18) have full outer shells and hardly react at all.
Key facts
- Group 1 metals have 1 outer electron, which they lose to form 1+ ions. Reactivity increases down the group because the outer electron is further from the nucleus and more easily lost.
- Alkali metals react with water to make a metal hydroxide and hydrogen; the solution is alkaline. Lithium fizzes; sodium melts into a ball; potassium burns with a lilac flame.
- Group 7 elements have 7 outer electrons and gain one to form 1− ions. They exist as diatomic molecules: F2, Cl2, Br2, I2.
- Down group 7, reactivity decreases, and melting and boiling points increase: chlorine is a green gas, bromine a red-brown liquid, iodine a grey-black solid.
- A more reactive halogen displaces a less reactive one from a solution of its salt: chlorine + potassium bromide → potassium chloride + bromine.
- Noble gases have full outer shells, so they are very unreactive and exist as single atoms. Their boiling points increase down the group.
- Uses: helium in balloons (light and does not burn), argon in light bulbs and welding (unreactive), neon in signs.