Chemical Formulas

chemical formula: a formula listing the number and type of each element in a molecule or a “formula unit.”  E.g., C₆H₁₂O₆.  Sometimes called the molecular formula.
A formula of C₆H₁₂O₆ means 6 atoms of carbon, 12 atoms of hydrogen, and 6 atoms of oxygen.

empirical formula: a chemical formula that has been reduced to lowest terms.  E.g., if the chemical formula is C₆H₁₂O₆ then the empirical formula would be C₁H₂O₁ or just CH₂O.

molecule: a group of elements that are chemically bonded together and behave as a single substance.

formula unit:  if a substance makes large crystals and not individual molecules, a formula unit is the “molecule” that would be represented by the empirical formula.

In a chemical formula, the subscript after a symbol indicates the number of atoms of that symbol.  If the subscript is after parentheses, it means that many of everything in parentheses.  For example, the formula Ca₃(PO₄)₂ means 3 atoms of calcium, and 2 groups of (1 phosphorus atom + 4 oxygen atoms).  This means we have a total of 2 phosphorus atoms and 8 (= 4 × 2) oxygen atoms in the molecule.