Ammonium is not found on the periodic table of elements because it is a compound rather than an element. Elements and compounds are pure chemical substances found in nature. The difference between an element and a compound is that an element is a substance made of the same type of atoms, whereas a compound is made of different elements in definite proportions. Examples of elements include iron, copper, hydrogen and oxygen. Examples of compounds include water (H2O) and salt (Sodium Chloride - NaCl)

Elements are listed according to their atomic number on the periodic table. As of 2016, there are 118 known elements.

The ratio of each element in a compound is usually expressed by a chemical formula. For example, water (H2O) is a compound consisting of two hydrogen atoms bonded to an oxygen atom. Ammonium has the chemical formula NH4(+) and is a compound consisting of one nitrogen atom and four hydrogen atoms.

Ammonium is an important source of nitrogen for many plant species, especially those growing on hypoxic soils. However, it is also toxic to most crop species and is rarely applied as a sole nitrogen source.

Germanium is a chemical element with the symbol Ge and atomic number 32.

Neptunium is a chemical element with the symbol Np and atomic number 93.

Americium is a synthetic radioactive chemical element with the symbol Am and atomic number 95.

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