The Periodic Table: Group Chemistry and Trends
This domain tests patterns in the main groups and the transition metals. You must describe physical properties, explain reactivity trends down a group and predict displacement reactions.
Group 1 alkali metals
Lithium, sodium and potassium are soft, low-density, shiny when freshly cut and stored in oil. They react with water to give a metal hydroxide and hydrogen, and with oxygen to form oxides. Reactivity increases down the group because the outer electron is further from the nucleus and more shielded, so it is lost more easily.
Group 7 halogens
Fluorine, chlorine, bromine and iodine exist as diatomic molecules. At room temperature chlorine is a pale green-yellow gas, bromine a red-brown liquid and iodine a grey-black solid. Reactivity decreases down the group because the outer shell is further from the nucleus, so the atom attracts an extra electron less strongly. A more reactive halogen displaces a less reactive halide from its salt solution.
Group 0 noble gases
Helium, neon and argon have full outer shells, so they are unreactive. They are gases at room temperature and are used where inert conditions are needed, such as in lamps and welding shields.
Transition metals
These are hard, dense, high-melting metals that often form coloured compounds, show variable valency and act as catalysts.
Worked example
Chlorine gas is bubbled through colourless potassium bromide solution. State the observation and explain it. The solution turns orange-brown as bromine is formed: Cl₂ + 2KBr → 2KCl + Br₂. Chlorine is above bromine in Group 7, so it is more reactive and displaces bromide. The same test with potassium chloride gives no reaction because chlorine cannot displace itself.
Traps that cost marks
- Saying Group 1 reactivity decreases down the group; it increases.
- Forgetting the states of the halogens at room temperature.
- Saying noble gases have no electrons; they have full outer shells.
- Predicting displacement when the halogen is lower in the group.