Why do the Group 1 alkali metals get more reactive down the group?
Group 1 the alkali metals: their physical properties, their reactions with water and oxygen, the trend in reactivity down the group, and the explanation in terms of electronic structure.
A focused answer to Edexcel GCSE Chemistry topic 6, covering the physical properties of the Group 1 alkali metals, their reactions with water and oxygen, the increasing reactivity down the group, and how the trend is explained by the increasing distance of the outer electron from the nucleus.
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What this dot point is asking
Edexcel wants you to describe the physical properties of the Group 1 alkali metals, their reactions with water and oxygen, the trend of increasing reactivity down the group, and to explain that trend in terms of electronic structure. The reactivity trend explained by electron shells is the key marking point.
Physical properties
The Group 1 metals are unusual for metals:
- They are soft and can be cut with a knife, showing a shiny surface that quickly dulls as it reacts with air.
- They have low densities (lithium, sodium and potassium float on water).
- They have low melting points for metals, and the melting point decreases down the group.
All Group 1 atoms have one electron in their outer shell, which they lose to form a ion.
Reactions with water
The alkali metals react with water to give a metal hydroxide and hydrogen:
For example, . The hydroxide dissolves to give an alkaline solution (turns universal indicator purple). The observations become more dramatic down the group:
- Lithium floats and fizzes steadily.
- Sodium melts into a ball, fizzes and skates across the surface.
- Potassium reacts so violently that the hydrogen ignites with a lilac flame.
Reactions with oxygen
The alkali metals react with oxygen to form metal oxides, for example . This is why they tarnish quickly in air and are stored under oil to keep out air and water.
The reactivity trend and its explanation
Reactivity increases down the group. The explanation is about how easily the single outer electron is lost:
Similar properties across the group
Because every Group 1 atom has the same number of outer electrons (one), they all react in the same way (losing that one electron), which is why they have similar chemical properties and form similar compounds.
Try this
Q1. State the number of electrons in the outer shell of a Group 1 atom. [1 mark]
- Cue. One.
Q2. Write the word equation for an alkali metal reacting with water. [2 marks]
- Cue. Metal plus water gives metal hydroxide plus hydrogen.
Q3. Explain why potassium is more reactive than sodium. [2 marks]
- Cue. Potassium's outer electron is in a shell further from the nucleus, so it is less strongly attracted and lost more easily, making the reaction faster.
Exam-style practice questions
Practice questions written in the style of Pearson Edexcel exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
Edexcel 20194 marksLithium, sodium and potassium are added to water. Describe what you would observe with potassium, write the balanced symbol equation for potassium reacting with water, and explain why potassium is more reactive than lithium.Show worked answer →
A 4-mark Group 1 reaction and trend question.
Potassium reacts vigorously: it floats, fizzes, melts into a ball and the hydrogen produced ignites with a lilac flame (1 mark for a good observation). The equation is (1 mark). Potassium is more reactive than lithium because its outer electron is in a shell further from the nucleus (1 mark), so the electron is less strongly attracted and is lost more easily, making the reaction faster (1 mark).
Markers reward the observation, the balanced equation, and the explanation in terms of the outer electron being further from the nucleus and more easily lost.
Edexcel 20213 marksGroup 1 metals all react in a similar way. Explain why they have similar chemical properties, and state the trend in their melting points down the group.Show worked answer →
A 3-mark properties-and-trend question.
The Group 1 metals all have one electron in their outer shell (1 mark), so they react in the same way, losing that one electron to form a ion, which gives them similar chemical properties (1 mark). Their melting points decrease down the group (1 mark).
Markers reward "one outer electron" as the reason for similar chemistry, and the decreasing melting point trend.
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Sources & how we know this
- Pearson Edexcel GCSE (9-1) Chemistry (1CH0) specification — Pearson Edexcel (2016)