A subscript tells the lowest whole number ratio of compounds in a given chemical reaction.

A subscript in a chemical formula represents the number of atoms of an element in a molecule. It indicates the ratio of atoms of each element present in the compound. For example, in water (H2O), the subscript 2 after hydrogen (H) indicates that there are two hydrogen atoms for every one oxygen atom in a molecule of water. This is the lowest whole number ratio of the elements in the compound.

Here are the steps to determine the ratio:

1. Identify the chemical formula. The formula should be written with the elements symbol and a subscript indicating the number of atoms.

2. Identify the subscript numbers. These are the numbers written at the lower right side of the element symbol.

3. Write down the ratio of the elements. The ratio is determined by the subscript numbers. For example, in H2O, the ratio is 2:1 (2 hydrogen atoms to 1 oxygen atom).

4. Simplify the ratio if necessary. If the ratio can be simplified to smaller whole numbers, then do so. However, in most cases, the chemical formula is already presented in the simplest whole number ratio.

Remember, if there is no subscript written after an element symbol, it is understood to be 1.