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The alkali metals are the elements of group 1, down the far left edge of the periodic table: lithium, sodium, potassium, rubidium, cesium and francium. They’re soft enough to cut with a knife, light enough (in lithium’s case) to float on water, and so reactive that none of them is ever found as a free metal in nature.
Hydrogen sits at the top of group 1 because it has one outer electron too, but it’s a nonmetal gas and isn’t counted as an alkali metal.
One outer electron
Every alkali metal has a single electron in its outermost s orbital:
- Li: [He] 2s¹
- Na: [Ne] 3s¹
- K: [Ar] 4s¹
- Cs: [Xe] 6s¹
That lone electron is far from the nucleus and well shielded by the inner shells, so it’s easily lost. Every alkali metal forms a +1 ion (Li⁺, Na⁺, K⁺…), and nearly all their chemistry comes from doing exactly that.
Physical properties
They’re unusual metals:
- Soft — you can slice sodium with a kitchen knife, revealing a shiny surface that dulls within seconds as it reacts with air.
- Low density — lithium, sodium and potassium are all less dense than water.
- Low melting points — because there’s only one electron per atom contributing to metallic bonding.
- Good conductors of heat and electricity.
Trends down the group (from our element data)
| Element | Melting point (°C) | Density (g/cm³) | First ionization energy (eV) | Electronegativity |
|---|---|---|---|---|
| Lithium | 180.5 | 0.534 | 5.39 | 0.98 |
| Sodium | 97.8 | 0.97 | 5.14 | 0.93 |
| Potassium | 63.4 | 0.89 | 4.34 | 0.82 |
| Rubidium | 39.3 | 1.53 | 4.18 | 0.82 |
| Cesium | 28.4 | 1.93 | 3.89 | 0.79 |
| Francium | ~27 (est.) | — | ~3.9 | 0.7 |
Going down the group:
- Atoms get bigger (an extra electron shell each time).
- Ionization energy falls, so the outer electron is easier to remove.
- Reactivity increases — dramatically.
- Melting points fall, because the metallic bonding weakens as atoms get larger.
- Density generally increases (potassium is the odd one out, slightly less dense than sodium).
Reaction with water
This is the classic demonstration. Every alkali metal reacts with water to form a hydroxide and hydrogen gas:
2M + 2H₂O → 2MOH + H₂
For sodium: 2Na + 2H₂O → 2NaOH + H₂
The solution left behind is strongly alkaline, which is where the group gets its name (alkali comes from the Arabic al-qalī, “plant ashes”, which were a traditional source of potassium and sodium compounds).
The violence increases down the group:
- Lithium fizzes steadily and floats.
- Sodium melts into a ball and whizzes across the surface.
- Potassium ignites the hydrogen with a lilac flame.
- Rubidium and cesium react explosively.
See the most reactive metal for why.
Reactions with oxygen and halogens
They tarnish rapidly in air, which is why they’re stored under oil. Burned in oxygen, they form oxides — lithium gives Li₂O, sodium mostly the peroxide Na₂O₂, and potassium and below form superoxides like KO₂. With halogens they form ionic salts: sodium plus chlorine gives sodium chloride, table salt.
Flame colours
Alkali metal compounds give characteristic flame colours, used in flame tests and fireworks:
| Element | Flame colour |
|---|---|
| Lithium | Crimson red |
| Sodium | Intense yellow-orange |
| Potassium | Lilac |
| Rubidium | Red-violet |
| Cesium | Blue-violet |
Rubidium and cesium were actually discovered in 1860–61 through their flame spectra by Robert Bunsen and Gustav Kirchhoff — cesium is named from the Latin caesius, “sky blue”, and rubidium from rubidus, “deep red”, after the colours of their spectral lines.
Uses
- Lithium — rechargeable batteries in phones, laptops and electric cars; lithium carbonate as a mood-stabilizing medicine; lightweight alloys.
- Sodium — sodium chloride (salt), sodium hydroxide (soap, paper), sodium vapour street lamps (the old orange glow), and as a coolant in some nuclear reactors.
- Potassium — potassium compounds are major fertilizers (the “K” in NPK); essential for nerve and muscle function in the body.
- Rubidium — atomic clocks, research.
- Cesium — the definition of the second is based on the cesium-133 atom; cesium atomic clocks underpin GPS and global timekeeping.
- Francium — none, beyond research: it’s too rare and radioactive.
Quick answers
Why are they called alkali metals? Because they react with water to form strongly alkaline hydroxide solutions.
Why are alkali metals stored in oil? To keep them away from air and moisture, which they react with quickly.
Is hydrogen an alkali metal? No. It shares group 1 because of its single electron, but it’s a nonmetal.
Explore
See the group on the periodic table and compare them side by side with the compare elements tool.
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