Why does the formula H2O contain a subscript 2 but no subscript 1?

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Two diagrams show the three-dimensional structure of a methane molecule: panel A uses a ball-and-stick model with a central carbon atom connected to four hydrogen atoms in a tetrahedral arrangement, while panel B illustrates the bond angle of 109.5 degrees and bond length of 1.09 angstroms between carbon and hydrogen atoms. — chemical compound Methane, in which four hydrogen atoms are bound to a single carbon atom, is an example of a basic chemical compound. The structures of chemical compounds are influenced by complex factors, such as bond angles and bond length.

In chemistry, a chemical formula uses subscripts to indicate the number of atoms of each element present in a compound. By convention, a subscript is only used when the number of atoms of a specific element is greater than one.1

Therefore, in the formula for water, $H_{2} O$ , the subscript 2 indicates that there are two atoms of hydrogen. Because there is only one atom of oxygen, no subscript is necessary; the absence of a subscript is understood to represent a single atom of that element.1