Alkali metals

Why they react so violently

Every alkali metal has a single electron in its outermost shell, sitting far from the nucleus and shielded by the full shells beneath it. Losing that electron gives the atom the stable electron arrangement of a noble gas, so it goes very easily. The result is a 1+ ion and a great deal of energy.

Drop lithium into water and it fizzes steadily. Sodium darts across the surface and may catch fire. Potassium bursts into lilac flame at once, and rubidium and caesium explode. In each case the metal produces hydrogen gas and a strongly alkaline hydroxide solution, which is where the group gets its name: “alkali” comes from the Arabic al-qalī, the ashes of plants from which potash and soda were first extracted.

What changes down the group

Going down group 1, each element adds a new electron shell. The outer electron sits further out and is held more loosely, so reactivity rises sharply. At the same time the metallic bonds holding the atoms together weaken: melting points fall from 181 °C for lithium to 28 °C for caesium, which melts on a warm day.

The metals also become denser and softer. Lithium, sodium and potassium float on water, while rubidium and caesium sink. All of them can be cut with a knife, revealing a bright silvery surface that tarnishes within seconds.

Storing and handling them

Because they react with oxygen and with moisture in the air, alkali metals are stored under mineral oil or in sealed ampoules filled with argon. Lithium is so light it floats on oil, so it is often coated in petroleum jelly instead. Fires involving these metals cannot be put out with water; they need dry sand or special powder extinguishers.

Where you meet them every day

Pure alkali metals are rare outside laboratories, but their compounds are everywhere. Sodium and potassium ions carry the electrical signals in your nerves, and your body works constantly to keep them in balance.

• Lithium: rechargeable batteries in phones, laptops and electric cars
• Sodium: table salt, baking soda, and the yellow glow of older street lamps
• Potassium: fertilisers, and the potassium in bananas and potatoes
• Rubidium and caesium: atomic clocks, including the caesium standard that defines the second