A chemical formula tells you the specific elements included in the compound and the number of atoms of each. The letters in a chemical formula are the symbols for the specific elements.

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Since every unique chemical substance has a definite composition, every such substance must be describable by a chemical formula. The well-known alcohol ethanol is composed of molecules containing two atoms of carbon, five atoms of hydrogen, and one atom of oxygen.

Compounds are represented by chemical formulas. Elements in a compound are represented by chemical symbols, and the ratio of different elements is represented by subscripts. There are different rules for writing the chemical formulas for ionic and covalent compounds.

If the chemical symbol has two letters the first letter is always capitalized and the second letter is written in lowercase. For chemical symbols that consist of one letter, that letter is always capitalized. For example, C represents carbon, Ca for calcium, O for oxygen.

Each element is given its own chemical symbol, like H for hydrogen or O for oxygen. Chemical symbols are usually one or two letters long. Every chemical symbol starts with a capital letter, with the second letter written in lower case. For example, Mg is the correct symbol for magnesium, but mg, mG and MG are wrong.

Certain rules apply to element symbols. The first letter is always capitalized, while the second (and third, for unverified elements) is lowercase. H is the chemical symbol for hydrogen. C is the chemical symbol for carbon.