Example 1: predicting a displacement
Will bromine displace iodine from potassium iodide?
- Bromine is above iodine in group 7.
- So bromine is more reactive.
- Yes: bromine + potassium iodide → potassium bromide + iodine.
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IB DemystifiedMYP Sciences
Groups and trends in the periodic table
Potassium bursts into lilac flames on water, bromine is a red-brown liquid, and argon quietly protects light bulb filaments. Elements in the same column of the periodic table behave like families, and their properties change in predictable ways as you go down each group.
Recommended for MYP 3 · About 3 lessons · Criteria A, B, C and D
On this page
- Learning objectives
- Before you start
- Key vocabulary
- Understanding the ideas
- Group trends at a glance
- Groups in the real world
- Worked examples
- Assessment tips
- Check your understanding
- Practice questions
- Investigation
- Criterion-linked questions
- Challenge questions
- Topic check
- Review your mistakes
- Your progress
Learning objectives
By the end of this topic you should be able to:
- describe the properties and reactions of the alkali metals (group 1) and their trends
- describe the properties of the halogens (group 7), their trends and displacement reactions
- explain why the noble gases (group 0) are unreactive and how they are used
- explain group properties and trends using electron arrangements
- analyse reaction data and design experiments
- discuss chlorinated water and lithium for batteries
Before you start
You will use these skills. If any feel shaky, review them first.
- atomic structure and electron arrangement (see Atomic structure)
- the periodic table (see Elements, compounds and the periodic table)
- acids and alkalis (see Acids, alkalis and indicators)
Key vocabulary
- Group
- A column of the periodic table; its elements have the same number of outer electrons.
- Alkali metals
- The reactive metals in group 1, which form alkalis with water.
- Halogens
- The reactive non-metals in group 7.
- Noble gases
- The unreactive elements in group 0, with full outer shells.
- Displacement
- A reaction in which a more reactive element takes the place of a less reactive one in a compound.
- Trend
- A regular change in a property going down a group or across a period.
Understanding the ideas
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Groups
Elements in the same group have the same number of electrons in their outer shell, so they react in similar ways. Going down a group, properties change gradually and predictably.
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Group 1: the alkali metals
The alkali metals (group 1) are soft, low-density metals that react with water to form an alkaline hydroxide and hydrogen gas. They lose their single outer electron when they react, and they become more reactive down the group because this electron is further from the nucleus and more easily lost.
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Group 7 and group 0
The halogens (group 7) are coloured non-metals that gain one electron when they react. They get darker and less reactive down the group; a more reactive halogen displaces a less reactive one from its compounds. The noble gases (group 0) have full outer shells, so they are very unreactive and are used where reactions must be avoided.
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Why does it matter?
Knowing these trends lets chemists predict the behaviour of elements they have never handled. It also explains uses such as chlorine for disinfecting water, helium in balloons and lithium in batteries.
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What does it connect to?
This topic builds on atomic structure and leads to ionic and covalent bonding, reactivity series and reactions in later chemistry.
Group trends at a glance
- Group 1: reactivity increases down; melting point decreases; react with water to give hydroxide + hydrogen.
- Group 7: reactivity decreases down; colour darkens; boiling point increases; more reactive halogens displace less reactive ones.
- Group 0: unreactive (full outer shells); boiling point increases down.
- Why: group 1 atoms lose an electron (easier further down); group 7 atoms gain an electron (harder further down).
Groups in the real world
Water treatment works add chlorine to kill bacteria. Lithium batteries power phones and electric vehicles. Argon fills light bulbs and is used in welding, and neon glows in advertising signs.
Worked examples
Example 2: a mean without the anomaly
Times of 20, 22, 35 and 21 s were recorded. Find the mean, leaving out the anomaly.
- 35 s is the anomaly.
- (20 + 22 + 21) ÷ 3 = 21 s.
Assessment tips
Questions on this topic often use tables of properties and reaction observations. Expect to:
- Describe trends down groups 1, 7 and 0.
- Predict properties of unfamiliar elements from trends.
- Explain trends using electron arrangements.
- Analyse reaction data and design safe experiments.
Common mistakes: saying reactivity increases down every group; confusing group 1 hydroxides with oxides; forgetting that a halogen cannot displace a more reactive one; and saying noble gases have no electrons in their outer shell.
Check your understanding
Quick questions on the ideas above. Try each one before using a hint.
Practice questions
Investigation: trends in group 2
Partially guided investigation · about 40 minutes · pairs
- Research question
- Is calcium more reactive than magnesium, as the group 1 trend would suggest for group 2?
- Scientific background
- Group 2 metals have two outer electrons. Like group 1, they may become more reactive further down the group.
- Hypothesis
- Write your own prediction, with a scientific justification.
- Variables
- Identify your independent, dependent and control variables, and explain how you will control them.
- Apparatus
- Magnesium ribbon, calcium granules (small), cold water, dilute hydrochloric acid (0.5 M), test tubes, test-tube rack, universal indicator, splints.
- Method
- Add a 1 cm piece of magnesium to cold water and a similar-sized calcium granule to another tube of cold water; compare the fizzing.
- Test the gas with a lit splint (squeaky pop) and the water with universal indicator.
- Repeat with dilute hydrochloric acid instead of water.
- Rank the metals in order of reactivity.
Safety. Wear eye protection. Use only small pieces. Calcium reactions produce an alkaline solution and heat; hydrochloric acid is an irritant. Keep flames away from large volumes of hydrogen.
Then evaluate: does group 2 follow the same trend as group 1, and how could you make the comparison fairer?
Criterion-linked questions
Criterion B: inquiring and designing
Criterion C: processing and evaluating
Criterion D: reflecting on the impacts of science
Challenge questions
Harder problems in unfamiliar contexts. Plan before you calculate.
Topic check
Five questions picked at random from the whole topic. Take a new set whenever you like.
Review your mistakes
Questions you got wrong on this device appear here so you can try them again. Answer one correctly and it leaves the list.
Your progress
Tracked separately for each skill, on this device only.
| Skill | Correct | Status |
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