What do the small numbers subscripts in the codes represent

A chemical subscript indicates how many atoms of the preceding element, or how many copies of a parenthesized group, appear in the formula unit being represented. An omitted subscript normally means one.

Meaning and context

In H₂O, the two applies to hydrogen and the unwritten one applies to oxygen. In Ca(OH)₂, the two multiplies the whole OH group, giving two oxygens and two hydrogens per represented formula unit.

Examples and usage

A coefficient is different. In 3H₂O, the three counts three water molecules or three corresponding amounts in an equation, multiplying the entire formula. The formula still describes water rather than a different compound.

What to check

Changing a subscript generally changes the substance or species being described. Balance an equation with coefficients rather than editing formulas. Also distinguish subscripts from superscripts used for charge or isotope notation.

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