IGCSE Chemistry: The Periodic Table Practice Questions
The Periodic Table arranges elements in order of increasing proton number. Elements in the same group have the same number of outer shell electrons and therefore react in similar ways, while the period number tells you how many occupied electron shells an atom has.
Topic 8 of Cambridge IGCSE Chemistry 0620 is mostly about explaining trends rather than recalling them. Almost every mark in this topic is earned by referring to outer shell electrons, shielding or the distance of the outer electron from the nucleus. The questions below follow the structure Cambridge uses for Paper 2 and Paper 4.
What you need to know for The Periodic Table
- Groups and periodsThe group number equals the number of outer shell electrons for the main groups. The period number equals the number of occupied electron shells.
- Group I, the alkali metalsSoft metals with one outer electron. Going down the group reactivity increases, density generally increases and melting point decreases. They react with water to give a metal hydroxide and hydrogen.
- Group VII, the halogensDiatomic non-metals with seven outer electrons. Going down the group colour darkens, density increases and reactivity decreases. A more reactive halogen displaces a less reactive one from its salt.
- Group VIII, the noble gasesMonatomic gases with a full outer shell, so they have almost no tendency to lose, gain or share electrons and are very unreactive.
- Transition elementsMetals between Groups II and III. They have high melting points and densities, form coloured compounds, show variable oxidation states and act as catalysts.
- Trend across a periodElements change from metallic on the left to non-metallic on the right, with the number of outer shell electrons increasing by one each step.
IGCSE Chemistry The Periodic Table questions and answers
4 exam-style questions written to the 0620 syllabus. Try each one on paper first, then open the worked answer to check your method against the marks.
Lithium and potassium are both in Group I. Explain, in terms of electronic structure, why they react in a similar way, and why potassium reacts more vigorously with water than lithium.
Show the worked answer
- Lithium has the electronic structure 2,1 and potassium has 2,8,8,1. Both have one electron in the outer shell.
- Chemical reactions involve outer shell electrons, so both lose one electron to form a 1+ ion and their reactions are similar.
- Potassium has more electron shells, so its outer electron is further from the nucleus and is shielded by more inner shells.
- The attraction between the nucleus and the outer electron is therefore weaker in potassium, so the electron is lost more easily and potassium reacts more vigorously.
Chlorine gas is bubbled through a colourless solution of potassium bromide. State what is observed and write the balanced equation for the reaction.
Show the worked answer
- Chlorine is above bromine in Group VII, so chlorine is the more reactive halogen.
- The more reactive halogen displaces the less reactive one from its salt, so chlorine displaces bromine.
- Bromine is formed in solution, which is orange or reddish brown, so the colourless solution changes colour.
- Balanced equation: Cl2 + 2KBr gives 2KCl + Br2.
Give two properties of transition elements that are not shown by the Group I metals.
Show the worked answer
- Transition elements form coloured compounds, for example copper(II) sulfate is blue. Group I compounds are typically white and give colourless solutions.
- Transition elements show more than one oxidation state, for example iron forms Fe2+ and Fe3+. Group I metals only ever form 1+ ions.
- Other acceptable answers: they act as catalysts, and they have much higher melting points and densities.
Explain why the noble gases in Group VIII are very unreactive.
Show the worked answer
- Helium has two outer electrons and the other noble gases have eight, which is a full outer shell.
- A full outer shell is a stable arrangement, so there is no tendency to lose, gain or share electrons.
- Since chemical bonding involves the transfer or sharing of outer electrons, the noble gases hardly react at all and exist as single atoms.
Common mistakes in this topic
- Confusing group with period. Groups are the vertical columns, periods are the horizontal rows.
- Explaining Group VII reactivity with the same reasoning as Group I. In Group VII reactivity decreases down the group because the outer shell is further from the nucleus and gains an electron less easily.
- Saying elements in a group have the same number of electrons. They have the same number of outer shell electrons.
- Giving the colour of the halogen gas when the question asks for the colour of the solution formed.
- Listing transition metal properties that Group I metals also have.
Exam tips
- Every trend answer in this topic should mention outer shell electrons, distance from the nucleus, and shielding by inner shells. Those three phrases carry most of the marks.
- Learn the halogen displacement results as a table of colours. Cambridge asks for observations, not just equations.
- Use the Periodic Table on the back of your data sheet to read off proton numbers and relative atomic masses instead of memorising them.
- When asked to predict a property of an element you have never met, state the trend first and then apply it.
- Group I reactions with water always give a metal hydroxide plus hydrogen. Learn that pattern once and it covers every member.
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The Periodic Table FAQs
Why does reactivity increase down Group I but decrease down Group VII?
Group I metals react by losing their single outer electron. Further down the group that electron is further from the nucleus and more shielded, so it is lost more easily and reactivity increases. Group VII non-metals react by gaining an electron. Further down the group the outer shell is further from the nucleus, so an incoming electron is attracted less strongly and reactivity decreases.
How do I work out the group and period from an electronic structure?
The number of electrons in the outer shell gives the group number for the main groups, and the number of occupied shells gives the period number. An atom with structure 2,8,7 has seven outer electrons and three shells, so it is in Group VII, Period 3, which makes it chlorine.
What do I need to know about transition elements for IGCSE Chemistry?
You need four things: high melting points and densities, coloured compounds, variable oxidation states, and use as catalysts. You should also be able to contrast each of these with the Group I metals, since that comparison is the usual exam question.
Are the noble gases completely unreactive?
For IGCSE purposes treat them as unreactive because they have a full outer electron shell. That is why they are used where an inert atmosphere is needed, such as argon in filament lamps and helium in balloons.
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Written to the published Cambridge IGCSE Chemistry (0620) syllabus. Check your school entry code and syllabus year, because Core and Extended candidates are assessed on different content. Last reviewed 2026-08-12.