Group 1 elements (the alkali metals) each have one valence electron, so they are very reactive and form ions with a +1 charge. They are soft, low-density metals that react with water to form an alkaline hydroxide and hydrogen gas.
Reactivity increases down the group because the atom gets larger and loses its valence electron more easily.
This page covers a single Form 4 content standard: Group 1, the alkali metals. It is a favourite exam topic because it brings together physical properties, a clear chemical reaction, and a trend you must explain in terms of atomic structure. Learn the properties and the reactivity trend here, and you will be ready for both the recall questions and the “explain the trend” questions that carry higher marks.
What the alkali metals are
Group 1 contains lithium (Li), sodium (Na), potassium (K) and the heavier metals rubidium, caesium and francium. They are called the alkali metals because they react with water to form alkaline (basic) hydroxide solutions. Every Group 1 atom has just one valence electron, which is the key to all their chemistry.
Physical properties
The alkali metals share a set of physical properties that are quite different from the metals you meet in everyday life:
- They are soft and can be cut with a knife.
- A freshly cut surface is shiny and silvery-grey, but it quickly turns dull because the metal reacts with oxygen in the air.
- They have low densities, lithium, sodium and potassium are light enough to float on water.
- They have low melting and boiling points compared with most metals.
- They are good conductors of heat and electricity.
Going down the group, the melting point decreases and the atomic size increases.
Chemical properties and reaction with water
Because each atom has one valence electron, an alkali metal reacts by losing that electron to form an ion with a +1 charge (for example Na becomes Na⁺). This makes the alkali metals very reactive, so they are stored under oil to keep air and water away from them.
Their most important reaction is with water. The metal reacts to produce a metal hydroxide, which is alkaline, and hydrogen gas. For sodium:
2Na + 2H₂O → 2NaOH + H₂
You should be able to describe what you see: the metal floats and moves about on the surface, it melts into a ball because the reaction gives out heat, it gets smaller and disappears, and gas bubbles are released. With potassium the reaction is more vigorous and the hydrogen often ignites with a lilac flame. The solution formed turns red litmus blue because it is alkaline.
The reactivity trend and its reason
The most examined idea is that reactivity increases down Group 1: potassium is more reactive than sodium, which is more reactive than lithium. You must be able to explain this:
Explanation. Going down the group, each atom has one more electron shell, so the atomic size increases. The single valence electron is therefore further from the nucleus and is held less strongly by the positive nucleus (it is also shielded by more inner shells). This means the valence electron is lost more easily, so the metal reacts more readily. That is why reactivity increases from lithium to potassium.
Worked example
Question. Sodium is placed in a trough of water. (a) Write a balanced chemical equation for the reaction. (b) Name the gas produced and state the type of solution formed. (c) Explain why potassium reacts more vigorously with water than sodium does.
Step 1, Write the equation. Sodium reacts with water to give sodium hydroxide and hydrogen: 2Na + 2H₂O → 2NaOH + H₂.
Step 2, Identify the products. The gas produced is hydrogen (H₂). The solution formed contains sodium hydroxide, which is alkaline.
Step 3, Compare the two metals. Potassium is below sodium in Group 1, so a potassium atom is larger than a sodium atom. Its valence electron is further from the nucleus and held less strongly, so it is lost more easily.
Step 4, Conclude. Because potassium loses its valence electron more easily, it reacts more readily, so the reaction with water is more vigorous than for sodium.
Answer. (a) 2Na + 2H₂O → 2NaOH + H₂. (b) Hydrogen gas; an alkaline solution. (c) Potassium is larger, so its valence electron is held less strongly and lost more easily, making it more reactive.
Practice question
Question. Lithium, sodium and potassium all react with water. (a) Arrange the three metals in order of increasing reactivity. (b) Explain the trend you have given.
Answer. (a) In order of increasing reactivity: lithium < sodium < potassium. (b) Going down the group the atomic size increases, so the valence electron is further from the nucleus and held less strongly. It is lost more easily, so reactivity increases from lithium up to potassium.
Exam tip
Two things score the marks here. First, when you describe the reaction with water, give observations (floats, moves, melts into a ball, dissolves, gas released) rather than just “it reacts”. Second, when you explain the reactivity trend, always mention atomic size increasing, the valence electron being further from the nucleus, and therefore lost more easily, a trend answer without this reasoning loses marks. Remember why they are called alkali metals: the hydroxide solution formed is alkaline and turns red litmus blue.
Where this fits
This standard follows straight on from groups and periods in the The Periodic Table of Elements chapter, and it pairs neatly with the opposite trend in Group 17. Revise the properties and equations with the revision notes and practise the reaction descriptions with the worked examples. In our online 1-to-1 SPM Chemistry lessons, taught in English, from RM50/hr, our teachers make sure your reaction observations are complete and your trend explanations always link atomic size to how easily the electron is lost, which is exactly what earns full marks on this standard across SPM Chemistry.
Quick recap
- Group 1 metals have 1 valence electron and form +1 ions.
- They are soft, low-density, silvery metals stored under oil.
- They react with water to give a metal hydroxide (alkaline) and hydrogen gas.
- Reactivity increases down the group because the atom is larger and loses its valence electron more easily.
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