Group trendsIB MYP Sciences: Revision notes
Section 1
Group 1: the alkali metals
Group 1 contains lithium, sodium and potassium (and others below). They are called the alkali metals because they form alkaline solutions with water. They are soft metals that can be cut with a knife, and they are shiny when freshly cut. Lithium, sodium and potassium have a low density and float on water. Each has one electron in its outer shell.
Section 2
Reactions of Group 1
With water: the metal fizzes, moves around on the surface and gives off hydrogen, leaving an alkaline solution of the metal hydroxide.
- lithium + water → lithium hydroxide + hydrogen
- 2Na + 2H₂O → 2NaOH + H₂
Lithium fizzes steadily. Sodium melts into a ball. Potassium melts, burns with a lilac flame and can spark.
With oxygen: the metals tarnish quickly in air and burn to form metal oxides, for example 4Li + O₂ → 2Li₂O.
Section 3
Reactivity trend, storage and uses
Reactivity increases down Group 1: lithium < sodium < potassium. The reactions with water get faster and more vigorous.
Because they react with oxygen and water vapour in the air, the alkali metals are stored under oil.
Uses: lithium is used in rechargeable batteries, sodium in some street lamps, and potassium compounds in fertilisers.
Do not say they are stored in oil to stop them melting or to keep them cold. It keeps air and water away.
Section 4
Group 7: the halogens
The halogens are in Group 7. They have 7 electrons in their outer shell. Their colour and state change down the group:
- fluorine: pale yellow gas
- chlorine: yellow-green gas
- bromine: red-brown liquid
- iodine: grey-black solid
The melting point increases down the group, which is why the states change from gas to liquid to solid. Halogens are poisonous and need careful handling.
Section 5
Halogen reactivity and displacement
Reactivity decreases down Group 7: fluorine is the most reactive and iodine the least. This is the opposite of Group 1.
A more reactive halogen displaces a less reactive halogen from a solution of its compound:
- chlorine + potassium bromide → potassium chloride + bromine
- Cl₂ + 2KBr → 2KCl + Br₂
The solution turns orange as bromine forms. Bromine does not displace chlorine, so there is no change.
Section 6
Group 0: the noble gases
The noble gases (helium, neon, argon and others) are in Group 0. Each has a full outer shell, so they are unreactive and exist as single atoms.
Uses that depend on being unreactive or less dense than air:
- helium: airships and balloons, because it is less dense than air and not flammable
- neon: bright signs, which glow red-orange when electricity passes through
- argon: filament lamps and welding, because it stops hot metal reacting with oxygen
That's the notes covered.
Carry on to the next subtopic.
Exam questions on Group trends
- A technician in Cape Town is preparing a demonstration of the alkali metals. She will drop small pieces of lithium, sodium and potassium into separate troughs of water. Before the lesson, she keeps the three metals in bottles of oil.Describe what the technician would see when a small piece of potassium is added to water, and how this differs from lithium.2 marks
- A chemistry class in Nairobi is investigating the halogens in Group 7. The teacher has sealed samples of chlorine, bromine and iodine, and solutions of potassium chloride, potassium bromide and potassium iodide. In one test she adds chlorine water to potassium bromide solution, and the colourless solution turns orange.Explain why the solution of potassium bromide turns orange when chlorine water is added.2 marks
- A student investigates how reactive lithium, sodium and potassium are with water. For each metal she puts 100 cm³ of water in a trough, adds a piece of the metal of mass 0.10 g, and records the time taken for the metal to disappear. She predicts that reactivity increases down Group 1. Her results: lithium 120 s, sodium 45 s, potassium 8 s.Identify the independent variable and the dependent variable in the investigation, and suggest one safety precaution.3 marks
Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).