An element that is an alkali metal is what?

very reactive

Explanation:

They form salts and alkali (acid-neutralizing) compounds such as baking soda. In pure form, they are very soft metals which catch fire on contact with water. (These metals are not found free in nature). These elements are Lithium(Li), Sodium(Na), Potassium(K), Rubidium(Rb), Cesium(Cs), and Francium(Fr)

all matter is made up of atoms

in any element all atoms are the same

elements in a compound are always in a constant ratio: apex