Groups 0, 1 and 7 – GCSE Combined Science Chemistry

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The periodic table is arranged in columns called groups. Elements in the same group behave in a similar way. This lesson covers three groups: the noble gases, the alkali metals and the halogens.

It matters because groups let you predict. If you know how sodium behaves, you can make a good guess about potassium. You do not need to learn every element one by one.

By the end you will be able to describe each group and write some key equations. You will predict how reactive an element is. Higher tier students will also explain the trends using electrons and the nucleus.

The noble gases

Group 0 contains helium, neon, argon, krypton, xenon and radon. Their atoms have full outer shells (2 electrons for helium, 8 for the others), so they have no need to gain, lose or share electrons. They are unreactive, or inert. Think of a bus with every seat taken: nobody needs to get on or off.

Because they do not bond, the noble gases exist as single atoms, which is called monatomic. They are colourless gases and do not burn. Helium is used in balloons because it is less dense than air and not flammable. Argon fills filament lamps because it will not react with the hot filament.

Boiling points increase down the group. Helium has the lowest and radon the highest.

There are only very weak forces between their atoms, so their boiling points are very low, and they rise down the group as the atoms get bigger: helium −269 °C, neon −246 °C and argon −186 °C. You can use the trend to predict, for example, that krypton boils at a temperature between argon and xenon.

The alkali metals

Group 1 contains lithium, sodium, potassium, rubidium and caesium. They are soft and can be cut with a knife. They have low melting points and low densities. Lithium, sodium and potassium float on water.

The melting point falls down the group. Their atoms have one outer electron, so they form 1+ ions. Their compounds are white solids that dissolve to make colourless solutions.

They react with air and water, so they are stored in oil.

Reactions of alkali metals

With water, an alkali metal makes a metal hydroxide and hydrogen: 2Na + 2H₂O → 2NaOH + H₂. The solution is alkaline, which is why the group is called the alkali metals.

Lithium fizzes steadily. Sodium melts into a ball and fizzes quickly. Potassium burns with a lilac flame. The reactions get more vigorous down the group.

They also react with chlorine to make metal chlorides: 2Na + Cl₂ → 2NaCl. They react with oxygen to make metal oxides: 4Li + O₂ → 2Li₂O.

Why Group 1 gets more reactive

A Group 1 atom reacts by losing its one outer electron. Think of a magnet and a paper clip. A clip close to the magnet is held tightly. A clip far away is held weakly and is easy to take off.

Down the group, the outer electron is further from the nucleus. There are also more shells in between. The pull from the nucleus is weaker, so the electron is lost more easily.

That is why potassium is more reactive than sodium, and sodium is more reactive than lithium.

The halogens

Group 7 contains fluorine, chlorine, bromine and iodine. Fluorine is a pale yellow gas and chlorine is a pale green gas. Bromine is a red-brown liquid. Iodine is a grey solid that gives a purple vapour. Melting and boiling points increase down the group.

Halogens exist as diatomic molecules, which means two atoms joined. They are F₂, Cl₂, Br₂ and I₂. They have seven outer electrons.

They react with metals to make ionic halides containing a 1− ion, such as 2Na + Cl₂ → 2NaCl. With non-metals they make covalent molecules such as HCl.

Displacement reactions

A more reactive halogen displaces a less reactive halogen from a solution of its salt. Think of a stronger player taking the place of a weaker one in a team.

Chlorine is more reactive than bromine. So Cl₂ + 2KBr → 2KCl + Br₂ happens, and the solution turns orange as bromine forms. Chlorine and bromine both displace iodine. Bromine cannot displace chlorine, so there is no reaction.

Reactivity decreases down Group 7. A halogen gains one electron, and the outer shell is further from the nucleus down the group. Higher tier: the pull on an extra electron is weaker, so it is gained less easily.

Recap

Group 0: full outer shells, unreactive, monatomic.

Group 1: one outer electron, soft metals that make a hydroxide and hydrogen with water. Reactivity increases down the group.

Group 7: diatomic non-metals with seven outer electrons. Reactivity decreases down the group.

A more reactive halogen displaces a less reactive one.