Arrhenius theorized that a base is a chemical compound that

Arrhenius theorized that a base is a chemical compound that contains hydroxide ions (OH⁻) and ionizes in water releasing them.

Explanation:

Svante Arrhenius was the first scientist to theorize about acids and bases.

Arrhenius' acid  base theory states that an acid is a compound that contains hydrogen atoms and ionizes in water to produce hydrogen ions (H⁺), and a base is a compound that contains hydroxide ions (OH⁻) that are released in water.

These are some examples of compounds which can be named as acid or base following Arrhenius' theory:

This theory is limited because it does not include compounds that do not contain or ionizes producting H⁺ or OH⁻. E.g. NH₃, which is a base (according to other theory).

alkaline metals are lithium, sodium, rubidium, potassium, cesium and francium.

alkali metals have electronegativity from 0.7 to 1, lowest in periodic table of elements, which means that alkali metals (i group in periodic table) has positive oxidation number in compounds.

electronegativity is a chemical property that describes the tendency of an atom to attract a shared pair of electrons towards itself.

alkaline metals (far left in main group) have lowest ionizations energy and easy remove valence electrons,

for example, potassium is the alkali metal and has a single valence electron in the outer electron shell.

electron configuration of potassium is: ₁₉k 1s²2s²2p⁶3s²3p⁶4s¹.

they can not exist alone in nature, because they are very reactive.