Portrait of Humphry Davy

Adapted from ⁠an image by Thomas Phillips (public domain), via Wikimedia Commons.

1778–1829

Humphry Davy

Discovered five elements between 1807 and 1808.

Elements

Davy made potassium with a battery in 1807 and watched the beads skate across water, burning lilac. Try it with any alkali metal.

Group 1 reactivity

Drop it in water

Click to drop it in

Ready to drop a small piece of potassium.

Melts, races around and bursts into a lilac flame straight away, crackling and spitting.

In water it
Floats0.862 g/cm³, water is 1
From the heat it
Melts into a ballmelts at 64 °C
How violent

2K + 2H₂O → 2KOH + H₂

Hydrogen gas fizzes off, and the metal hydroxide dissolves, making the water alkaline: a few drops of phenolphthalein indicator turn it pink.

Why it gets more violent down the group

Every alkali metal has one electron in its outer shell, and reacting means giving it away. Further down the group that electron sits in a shell further from the nucleus, shielded by more electrons inside, so it takes less energy to remove and the reaction runs faster.

  • Lithium520 kJ/mol
  • Sodium496 kJ/mol
  • Potassium419 kJ/mol
  • Rubidium403 kJ/mol
  • Caesium376 kJ/mol

Energy to remove the outer electron (first ionization energy, NIST). Speed isn’t the same as energy: per atom, lithium’s reaction releases slightly more energy than sodium’s, it just releases it slowly. Francium’s value (393 kJ/mol) is a little higher than caesium’s, because of relativistic effects, so it may not be the most violent after all.

Don’t try this at home. Demonstrations use rice-grain-sized pieces behind safety screens; the heavier metals are only shown on video.

Keep exploring

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