Which element has the chemical symbol K?

Potassium, and the K is from kalium, the Latinised form of an Arabic word for the plant ash that potassium carbonate was extracted from. The English name comes from the same substance by a different route - pot ash, the residue left when you burn wood and leach the ashes in a pot.

It was first isolated in 1807 by Humphry Davy, who did it by running electricity through molten potash, and it was the first metal ever obtained by electrolysis. He reportedly danced around the laboratory when he saw the globules form. He isolated sodium days later by the same method and went on to identify calcium, magnesium, barium, strontium and boron - six elements in about two years.

In the body it does the other half of the job sodium does. Nerve and muscle cells maintain a voltage across their membranes by keeping potassium high inside and sodium high outside, and every nerve impulse and every heartbeat is that gradient briefly collapsing and being restored. Which is why potassium levels are watched so carefully in hospital: too much or too little stops the heart, and potassium chloride is what is used in lethal injections for exactly that reason.