Edexcel GCSE Chemistry (1CH0) · Topic 6 — Groups in the periodic table
Mini-Lesson
Groups in the Periodic Table
This mini-lesson walks you through the whole of Edexcel Topic 6: the alkali metals (Group 1), the halogens (Group 7) and the noble gases (Group 0) — their properties, their reactions, and the trends as you go down each group.
Make a guess: why do alkali metals get more reactive further down?
Mission 1Why are Group 1 metals not equally reactive?
Mission 2Why does displacement only go one way?
Work through each screen, answer the questions as you go and collect ⭐ stars. Press Start when you're ready.
Reading the table
Groups share an outer shell
Elements are arranged by increasing atomic number. For Groups 1 to 7, the group number (the column) tells you the number of electrons in the outer shell — and that controls how an element reacts.
Group 1 — the alkali metals: 1 outer electron.
Group 7 — the halogens: 7 outer electrons.
Group 0 — the noble gases: a full outer shell (2 or 8).
Why it matters: you can classify an element as an alkali metal, halogen or noble gas just from its position in the table — because position fixes the outer-shell electron count.
Quick check
Which group?
?An element sits in the far-right column of the periodic table and its atoms have a full outer shell of electrons. Which group does it belong to?
Group 1 · alkali metals
The alkali metals
Lithium, sodium and potassium are soft (you can cut them with a knife) and have relatively low melting points for metals. They are stored under oil because they react quickly with air and water.
Each atom has 1 electron in its outer shell.
They react by losing that 1 electron to form a +1 ion.
With water they fizz, releasing hydrogen and forming a metal hydroxide (an alkaline solution).
metal + water → metal hydroxide + hydrogene.g. 2Na + 2H₂O → 2NaOH + H₂ (the solution turns universal indicator purple)
Group 1 · the trend
Reactivity increases down the group
Going down Group 1, the reactions with water get more vigorous: lithium fizzes gently, sodium darts about and melts into a ball, potassium ignites the hydrogen with a lilac flame.
Reactivity with water increases down Group 1: Li < Na < K. You can use this to predict that rubidium and caesium are even more reactive.
Why? Down the group the outer electron is in a shell further from the nucleus, with more inner shells shielding it. The nuclear pull on that electron is weaker, so it is lost more easily — making the metal more reactive.
Quick check
Predict the trend
?Caesium (Cs) sits below potassium in Group 1. Predict how it will react when dropped into water, and explain why.
Mission 1 check
Why are Group 1 metals not equally reactive?
?Lithium and potassium are both alkali metals. Potassium is lower in the group. Dropped into water, potassium reacts far more violently. Which statement explains why?
Mission 2 of 2
Why does displacement only go one way?
Make a guess: why does halogen displacement only go one way?
Group 7 · halogens
The halogens and their colours
The halogens are non-metals that exist as diatomic molecules (Cl₂, Br₂, I₂). You must recall their colours and states at room temperature:
Down the group the colour gets darker and melting/boiling points rise: the state changes gas → liquid → solid.
Predicting: fluorine (above chlorine) is a pale-yellow gas; astatine (below iodine) would be an even darker solid. The test for chlorine: it bleaches damp blue litmus paper white.
Match it
Name that halogen
Tap a halogen on the left, then its colour & state on the right.
Group 7 · reactions
How halogens react
A halogen reacts by gaining 1 electron to complete its outer shell, forming a −1 ion (a halide). Two reactions you must know:
With metals → a metal halide (an ionic salt). e.g. sodium + chlorine → sodium chloride: 2Na + Cl₂ → 2NaCl.
With hydrogen → a hydrogen halide. e.g. H₂ + Cl₂ → 2HCl. Hydrogen halides dissolve in water to form acidic solutions (HCl → hydrochloric acid).
halogen + metal → metal halidehalogen + hydrogen → hydrogen halide (dissolves to give an acid)
Predicting: the same patterns let you predict the products for other halogens — e.g. bromine + potassium → potassium bromide.
Group 7 · displacement
Displacement reactions
A more reactive halogen will displace a less reactive halogen from a solution of its salt (a halide).
Cl₂ + 2KBr → 2KCl + Br₂. Chlorine (more reactive) pushes out bromine, turning the solution orange. Iodine would not displace chlorine — it is less reactive.
Reactivity order: Cl > Br > I (reactivity decreases down Group 7). So chlorine also displaces iodine from potassium iodide: Cl₂ + 2KI → 2KCl + I₂. Use this to predict: astatine (At) is the least reactive, so it cannot displace any of the others.
Quick check
Will it displace?
?Bromine water is added to a colourless solution of potassium chloride (KCl). What happens?
Group 7 · the trend
Reactivity decreases down the group
Halogens react by gaining an electron. The easier it is to attract an electron, the more reactive the halogen — and this gets harder going down the group.
Down Group 7 the atom gets bigger: the outer shell is further from the nucleus and more shielded, so the nucleus attracts an incoming electron less strongly — reactivity falls.
Contrast: Group 1 reactivity increases down the group (losing an electron gets easier), but Group 7 reactivity decreases down the group (gaining an electron gets harder). Same cause — bigger atoms, weaker pull — opposite effect.
Quick check
Explain the halogen trend
?Why is chlorine more reactive than iodine?
Higher Tier only
Displacement as redox (HT)
Higher-tier students must explain why halogen displacement reactions are redox reactions, in terms of electron transfer. Take chlorine displacing bromine:
Chlorine is reduced — each Cl atom gains an electron (Cl₂ → 2Cl⁻).
Bromide is oxidised — each Br⁻ loses an electron (2Br⁻ → Br₂).
Remember OIL RIG: Oxidation Is Loss, Reduction Is Gain (of electrons). The more reactive halogen is the one that is reduced — it takes the electrons.
Quick check · HT
Oxidised or reduced?
?In the reaction Cl₂ + 2KBr → 2KCl + Br₂, what happens to the bromide ions (Br⁻)?
Group 0 · noble gases
The noble gases are inert
The noble gases (helium, neon, argon, krypton…) are chemically inert — they barely react at all. The reason is their electronic configuration:
Neon has a full outer shell (8 electrons). With nothing to gain by reacting, noble gases are unreactive.
Uses follow from inertness: helium is used in balloons/airships (low density, non-flammable); argon fills light bulbs and shields welds (won't react with the hot metal). Their uses depend on being inert, low-density and/or non-flammable.
Quick check
Why so unreactive?
?Why are the noble gases chemically inert compared with other elements?
Mission 2 check
Why does displacement only go one way?
?Bromine water is added to potassium iodide solution, and iodine forms. Iodine is added to potassium bromide solution, and nothing happens. Why does this displacement only go one way?
Group 0 · physical trends
Trends down the noble gases
Even unreactive elements show clear physical trends. Going down Group 0, both boiling point and densityincrease:
Boiling point increases down Group 0 (He has the lowest). Density rises too. You can use this to predict that xenon boils higher and is denser than krypton.
Predicting: because the trend is smooth, an unknown noble gas below krypton would have a higher boiling point and a greater density than krypton.
Quick check
Predict for xenon
?Xenon (Xe) lies below krypton in Group 0. Compared with krypton, xenon should have a…
Sort it
Which way does reactivity go?
Tap a statement, then the box for the group it describes.
⬇ Group 1 — reactivity rises
⬇ Group 7 — reactivity falls
Recap
The big ideas to know
Group 1 (alkali metals): soft, low m.p.; react with water → hydroxide + H₂; reactivity increases down the group (outer e⁻ lost more easily).
Group 7 (halogens): diatomic; Cl₂ yellow-green gas, Br₂ orange-brown liquid, I₂ grey-black solid; react with metals → metal halides and with H₂ → hydrogen halides (acidic).
Displacement: a more reactive halogen displaces a less reactive one (Cl > Br > I); reactivity decreases down the group (incoming e⁻ gained less easily).
HT: displacement is redox — the displacing halogen is reduced (gains e⁻), the halide is oxidised (loses e⁻).
Group 0 (noble gases): inert — full outer shells; boiling point and density increase down the group.
You've covered the key ideas in Edexcel Topic 6 — Groups 1, 7 and 0. Press Finish to see your score.
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Mini-lesson complete!
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You've worked through Groups in the Periodic Table for Edexcel GCSE Chemistry. 🎉