The key idea
Elements in the same group have the same number of outer electrons and similar chemical properties.The periodic table is arranged in order of increasing atomic number.
Use the labels to explain the scientific relationship shown.
The bit that matters
Keep the idea tight, then use the worked example to practise the exact exam wording.
Structure of the periodic table
The periodic table arranges all known elements in order of increasing atomic number.Elements in the same vertical column (group) share the same number of electrons in their outer shell and have similar chemical properties.Elements in the same horizontal row (period) have the same number of electron shells.
History of the periodic table
Early attempts at classifying elements, such as Newlands' law of octaves, arranged elements by atomic mass but placed dissimilar elements in the same group.Mendeleev improved this by leaving gaps for undiscovered elements and predicting their properties.When these predictions were confirmed, the periodic table gained wide acceptance.The modern table is arranged by atomic number, not atomic mass.
Group 1 — alkali metals
The alkali metals (Li, Na, K, Rb, Cs, Fr) all have one electron in their outer shell.They react vigorously with water to produce a metal hydroxide and hydrogen gas, and they lose their outer electron easily to form +1 ions.Reactivity increases down the group as the outer electron becomes easier to remove.
Group 7 — halogens
The halogens (F, Cl, Br, I, At) all have seven electrons in their outer shell and form −1 ions. g.Cl₂) and are oxidising agents that react with metals to form salts called halides.Reactivity decreases down the group as the outer shell becomes further from the nucleus and harder to fill.
Definitions to learn
Period
A horizontal row of elements in the periodic table; all elements in a period have the same number of electron shells.
Group
A vertical column of elements in the periodic table; all elements in a group have the same number of outer electrons.
Alkali metals
The elements in Group 1 of the periodic table, which have one outer electron and react vigorously with water.
Halogens
The elements in Group 7 of the periodic table, which have seven outer electrons and form −1 ions.
Noble gases
The elements in Group 0 with full outer shells, making them very unreactive.
Transition metals
The block of metals in the middle of the periodic table, which form coloured compounds and can have variable oxidation states.
Explain why potassium is placed in Group 1 of the periodic table.
Potassium has the electronic structure 2, 8, 8, 1.
It has one electron in its outer shell.
Elements with one outer electron are in Group 1.
Potassium is in Group 1 because it has one electron in its outer shell.
In periodic table questions, always link the group number to the number of outer electrons and the period number to the number of electron shells.
Do not say elements are arranged in order of atomic mass. The modern table uses atomic number.
How to score full marks
- 1State the group number AND the number of outer electrons together — e.g. 'Group 1 because it has 1 outer electron'.
- 2For reactivity trends: Group 1 increases DOWN (easier to lose e⁻); Group 7 decreases DOWN (harder to gain e⁻) — these are opposite trends.
- 3Mendeleev's key contribution was leaving GAPS — always mention this when explaining why his table was accepted.
Try these yourself
Open each answer only after you have explained the full chemical process.
1State what is meant by a period in the periodic table.[1 mark]
- 1.Think about the rows.
2Give two properties of the noble gases that explain why they are unreactive.[2 marks]
- 1.Consider their electronic structures.
3Explain why elements in the same group have similar chemical properties.[2 marks]
- 1.Link to outer electrons.
4State one limitation of Newlands' law of octaves.[2 marks]
- 1.Think about properties of the elements placed in the same group.
5Explain how Mendeleev's periodic table was different from Newlands' and why it was more successful.[4 marks]
- 1.Consider gaps and predictions.
6Describe the trend in reactivity as you go down Group 1.[3 marks]
- 1.Link to ionisation energy or ease of losing an electron.
7Explain why the atomic radius increases down Group 1.[2 marks]
- 1.Link to number of electron shells.
8Explain why elements in Group 7 become less reactive going down the group.[3 marks]
- 1.Compare ability to gain an electron.