Subscript
A small number written below and to the right of a chemical symbol in a formula. It tells you how many atoms of that element are in one formula unit. When no subscript is written, the number of atoms is 1.
Example: In CO₂ the subscript 2 means there are 2 oxygen atoms bonded to 1 carbon atom.
Coefficient
A large number written in front of a chemical formula in an equation. It multiplies everything in the formula and shows how many units of that substance are involved in the reaction.
Example: 3H₂O means 3 molecules of water — that is 6 hydrogen atoms and 3 oxygen atoms in total.
Exam tip: Subscripts are part of the formula and cannot be changed when balancing. Only coefficients can be changed.