A chemical formula provides a concise, standardized way to represent a molecule or compound using element symbols. The small, lowered number written immediately following an element’s symbol is called a subscript. This subscript details the precise atomic composition of the substance. It appears at the bottom right of the element symbol it modifies, making it visually distinct from other numbers in chemical notation.
The Count: What Subscripts Represent
The primary function of a subscript is to indicate the exact number of atoms of the element directly preceding it within a single molecule or formula unit. Subscripts reflect the specific ratio in which elements must combine to form that substance. For example, the formula for water, \(\text{H}_2\text{O}\), uses the subscript ‘2’ to show that every molecule contains two atoms of hydrogen.
In carbon dioxide, \(\text{CO}_2\), the subscript ‘2’ applies only to the oxygen atom, meaning the molecule contains one carbon atom and two oxygen atoms. Changing a subscript results in a completely different substance with distinct chemical properties, such as changing \(\text{H}_2\text{O}\) to \(\text{H}_2\text{O}_2\) (hydrogen peroxide).
Interpreting Implied Subscripts and Parentheses
If an element symbol is written without any visible subscript, the number is understood to be one. For example, in sodium chloride (\(\text{NaCl}\)), the absence of a subscript next to sodium (\(\text{Na}\)) or chlorine (\(\text{Cl}\)) indicates a 1:1 ratio of one sodium atom and one chlorine atom.
Parentheses are used to group polyatomic ions when more than one group is required. The subscript written outside the closing parenthesis acts as a multiplier for every atom inside the parentheses. In calcium hydroxide, \(\text{Ca}(\text{OH})_2\), the subscript ‘2’ means there are two hydroxide groups. This results in one calcium atom, two oxygen atoms, and two hydrogen atoms.
Subscripts Compared to Coefficients and Superscripts
A coefficient is a large number placed in front of the entire chemical formula, such as the ‘2’ in \(2\text{H}_2\text{O}\). Coefficients indicate the number of individual molecules or moles of the substance. They are adjusted when balancing chemical equations to satisfy the law of conservation of mass, but changing a coefficient only changes the quantity of the substance, not its chemical identity.
In contrast, a superscript is a small number written to the upper right of an element or molecule, used to denote an electrical charge. Superscripts are reserved for ions, indicating the number of electrons an atom has gained or lost. For example, \(\text{Ca}^{2+}\) indicates a calcium ion with a positive charge of two. Only the subscript defines the fixed, internal atomic composition of the molecule itself.