Edexcel IGCSE Chemistry · Spec 2.1-2.4C
Group 1: The Alkali Metals
The properties, reactions and trends of the Group 1 alkali metals.
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Group 1: The Alkali Metals
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Explained
The alkali metals, and why they get more reactive downwards
Group 1 is lithium, sodium, potassium, rubidium, caesium and francium. They are metals, but not the sort most people picture: they are soft enough to cut with a knife, and lithium, sodium and potassium are less dense than water, so they float on it.
They all have one electron in their outer shell, which is what makes them a group, and losing that one electron to form a 1 plus ion is the whole of their chemistry.
Reaction with water
Each reacts with water to give a metal hydroxide and hydrogen. Lithium gives lithium hydroxide and hydrogen, and the hydroxide dissolves to make an alkaline solution, which is where the group's name comes from.
What you actually see gets more dramatic down the group. Lithium fizzes steadily and floats. Sodium fizzes vigorously, melts into a ball from the heat released, and skims across the surface. Potassium does all of that and the hydrogen ignites with a lilac flame.
The trend, and why
Reactivity increases down the group, because reacting means losing the outer electron and that gets easier further down.
Two reasons work together. The outer electron is in a shell further from the nucleus, so the attraction between them is weaker. And there are more inner shells between them, which shield the outer electron from the nuclear charge.
So the outer electron is held less strongly, is lost more easily, and the metal is more reactive. State both reasons and then the consequence, because they are marked separately.
What the mark scheme accepts and rejects
An Edexcel International GCSE Chemistry mark scheme asks what you would observe when lithium is added to water. It credits effervescence, fizzing or bubbles, the metal moving, the metal floating, and the metal getting smaller or disappearing. It notes that moving on the surface earns both the moving and floating marks.
Then it rules three things out: ignore gas given off, ignore heat produced, and ignore colour change.
Gas given off is a conclusion rather than an observation, and the same distinction appears throughout chemistry. Heat produced is not something you see. And there is no colour change to observe, so writing one describes something that did not happen.
On the trend, another mark scheme wants the outer electron being more shielded from the nuclear attraction and therefore more easily lost. Its notes reject more outer shells as an explanation, require the final point to be comparative, and specify the singular: it will not allow electrons in the mark about the one being lost.
The singular matters because a Group 1 atom loses exactly one electron. Writing electrons suggests you do not know how many, which is the thing the group is defined by.
Storage and safety
Alkali metals are stored under oil, because they react with both the oxygen and the water vapour in air. A freshly cut surface is shiny and tarnishes within seconds as it reacts with oxygen, which is a demonstration worth remembering as evidence of how reactive they are.
Predicting an unfamiliar member
Questions often ask about rubidium or caesium, which you will not have seen. Use the trends: further down means more reactive, lower melting point, higher density, and a more violent reaction with water producing the hydroxide and hydrogen.
Predicting from the pattern is the skill being tested, and saying which trend you are using is part of the answer.
Spec 2.1-2.4C
What you need to know
- Describe the properties of the alkali metals
- Describe how they react with water
- Explain the trend in reactivity down the group
Active recall
Quick check
Answer each question before opening the answer.
How does reactivity change down Group 1, and why?
It increases down the group because the outer electron is further from the nucleus and is lost more easily.
What is formed when an alkali metal reacts with water?
A metal hydroxide (alkaline solution) plus hydrogen gas.
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