(a) Group 1 (alkali metals) – lithium, sodium and potassium
- Syllabus
- 2024
- Topic
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- Level
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Learning objectives
2.1Group 1 reactions with waterUnderstand how the similarities in the reactions of these elements with water provide evidence for their recognition as a family of elements2.2Group 1 reactivity trendUnderstand how the differences between the reactions of these elements with air and water provide evidence for the trend in reactivity in Group 12.3Predicting alkali metal propertiesUse knowledge of trends in Group 1 to predict the properties of other alkali metals24C Group 1 electronic configurationsExplain the trend in reactivity in Group 1 in terms of electronic configurations
Recognise the shared Group 1 reaction with water
Lithium, sodium and potassium form one family because each reacts with water in the same chemical pattern: the products are hydrogen gas and a solution of the metal hydroxide.
\ce{2M(s) + 2H2O(l) -> 2MOH(aq) + H2(g)}
| Shared evidence | What it shows |
|---|---|
| bubbles or fizzing | hydrogen gas is produced |
| the metal moves and gets smaller | the metal is reacting and being used up |
| the final colourless solution is alkaline | a soluble metal hydroxide has formed |
The shared products and reaction pattern identify the family; the speed and intensity do not have to be identical. Melting into a ball or producing a flame is not a reliable observation for all three metals.
Use observations to establish the Group 1 reactivity trend
Reactivity increases down Group 1: lithium is the least reactive of these three metals, sodium is more reactive, and potassium is the most reactive.
| Metal | Reaction with water | Evidence from air |
|---|---|---|
| lithium | fizzes and moves; usually keeps its solid shape | tarnishes, but least rapidly of the three |
| sodium | reacts faster and usually melts into a moving ball | tarnishes faster than lithium |
| potassium | reacts very vigorously and may ignite with a lilac flame | reacts most readily, so it must be kept away from air and moisture |
The comparison is valid because the metals undergo similar types of reaction but at different rates or intensities. Faster fizzing, quicker disappearance, easier melting or ignition and more rapid reaction with air are evidence of greater reactivity.
A flame colour alone identifies a particular metal; it does not define reactivity. Compare how readily and vigorously the metals react under similar conditions. These reactive metals are stored under oil to prevent contact with oxygen and water vapour.
Predict the behaviour of other alkali metals
Use the established Group 1 pattern to predict an unfamiliar alkali metal: keep the family's shared chemical behaviour, then extend the trend in the correct direction.
| Position of unfamiliar metal | Prediction |
|---|---|
| below potassium, such as rubidium or caesium | more reactive than potassium; reacts with water even faster and more violently |
| anywhere in Group 1 | forms a +1 ion and reacts with water to form hydrogen and a metal hydroxide |
For caesium, predict the same broad observations as potassium—fizzing, movement and an alkaline solution—but a still faster and more violent reaction. The general equation remains 2M+2HX2O2MOH+HX2.
A trend supports a comparative prediction, not an invented exact value. State 'more reactive than potassium' or 'reacts more vigorously', unless numerical data are supplied; do not guess a precise rate, temperature or flame colour.
Explain the Group 1 trend from electronic configuration
Every Group 1 atom has one electron in its outer shell. It reacts by losing that electron to form a +1 ion, so the ease of electron loss controls its reactivity.
| Atom | Electronic configuration | Occupied shells | Relative ease of losing the outer electron |
|---|---|---|---|
| lithium | 2,1 | 2 | hardest of these three |
| sodium | 2,8,1 | 3 | easier |
| potassium | 2,8,8,1 | 4 | easiest |
Down the group, atoms have more occupied electron shells. The outer electron is farther from the nucleus and is more shielded by inner electrons, so its attraction to the nucleus is weaker. It is therefore lost more easily, making reactions more rapid and vigorous down the group.
Although nuclear charge also increases down the group, increased distance and shielding outweigh it for the outer electron. The explanation must end with easier electron loss; simply saying that potassium has more electrons does not explain greater reactivity.