Summary
Highlights
Alkali metals, including lithium, sodium, potassium, rubidium, caesium, and francium, differ from other metals. They are soft, have low densities, and low melting points, unlike typical metals. They are also highly reactive and react vigorously with water, oxygen, and group 7 elements. Reactivity increases down the group, while melting and boiling points decrease.
Alkali metals are highly reactive because they have one electron in their outermost shell. To achieve stability, they readily lose this single electron, becoming a 1+ ion. As you go down the group, the atomic radius increases, meaning the outermost electron is further from the positive nucleus and therefore more easily lost, increasing reactivity. Reactivity is simply how easily atoms lose or gain electrons.
Because alkali metals easily lose their outer electrons, they almost always form ionic compounds with nonmetals. This involves a metal atom donating an electron to a nonmetal, creating oppositely charged ions that are attracted by electrostatic forces (ionic bond). A common example is sodium chloride (table salt). These compounds are generally white solids and dissolve in water to form colorless solutions.
Alkali metals react vigorously with water to produce a metal hydroxide and hydrogen gas. Reactivity increases down the group, with potassium and subsequent elements releasing enough energy to ignite the hydrogen gas. When heated in chlorine gas, they form white metal chloride salts, with reactions becoming more vigorous down the group. With oxygen, they form metal oxides, though the specific type of oxide (e.g., oxide, peroxide) depends on the particular metal.