The simplest unit of an element is the atom. The atom is never broken down into smaller particles in chemical reactions. The relative mass of an atom is given by its atomic mass, which appears on the periodic table for each element. An amount of an element equal to its atomic mass in grams is one mole of the element.
Atoms combine in definite whole number ratios to form compounds. Water (H 2O) is composed of two atoms of hydrogen (H) and one atom of oxygen (O). One mole of water is about 18.0 grams of water, an amount equal to the sum of the atomic masses of two hydrogen atoms and one oxygen atom in grams: 1.00 g + 1.00 g + 16.0 g = 18.0 g. Speaking in terms of moles is common when describing amounts of substances.
The chemical reaction between two or more substances is described using a chemical equation. The reaction between hydrogen and oxygen to form water is shown as
2H 2 ( g) + O 2 ( g) → 2H 2O ( l)