Group 1, Group 7 and Group 0 · GCSE Chemistry

Group 1: the alkali metals

The alkali metals (lithium, sodium, potassium…) have one electron in their outer shell. They are soft metals with low density.
They react with water to form a metal hydroxide and hydrogen: 2Na + 2H₂O → 2NaOH + H₂. The solution is alkaline.
They react with oxygen to form metal oxides, and with chlorine to form metal chlorides.
Reactivity increases down the group: the outer electron is further from the nucleus and more shielded, so it is attracted less strongly and lost more easily.
With water: lithium fizzes steadily; sodium melts into a ball and moves quickly; potassium burns with a lilac flame.

The halogens are non-metals that exist as molecules of two atoms (diatomic), such as Cl₂ and Br₂.
Down the group, relative molecular mass, melting points and boiling points increase. Fluorine is a gas, bromine a liquid and iodine a solid at room temperature.
They react with metals to form ionic compounds in which the halide ion has a −1 charge, and form covalent compounds with non-metals.
Reactivity decreases down the group: the outer shell is further from the nucleus, so it is harder to gain an electron.

A more reactive halogen displaces a less reactive halogen from an aqueous solution of its salt.
Chlorine + potassium bromide → potassium chloride + bromine (the solution turns orange).
Bromine does not react with potassium chloride, because bromine is less reactive than chlorine.

The noble gases (helium, neon, argon…) are unreactive and do not easily form molecules, because they have stable electron arrangements: 8 outer electrons, or 2 for helium.
Boiling points increase down the group as relative atomic mass increases.

Alkali metals: The group 1 elements, which react with water to form alkaline solutions.
Halogens: The group 7 elements, which exist as diatomic molecules.
Noble gases: The unreactive group 0 elements with full outer shells.
Diatomic: Made of molecules containing two atoms, such as Cl₂.
Halide ion: A negative ion with a −1 charge formed from a halogen, such as Cl⁻.
Displacement reaction: A reaction in which a more reactive element takes the place of a less reactive one in a compound.
Metal hydroxide: A compound formed, along with hydrogen, when an alkali metal reacts with water.