Chemistry

A+...subscript.

That is the subscript in a chemical formula. For example, oxygen by itself in a jar is usually diatomic (except for ozone, but that doesn't last long in standard temperatures and pressures) so its molecular formula is O2, and so is hydrogen H2, water molecule has one oxygen bonded to two hydrogen H2O, so when we write the formula for making water it has to take into account that each molecule of oxygen forms two molecules of water which needs two molecules of hydrogen (remember the least common multiple stuff we went through in grade school?) and the energy it has to take from its immediate viscinity

O2 + 2H2 + ΔE = 2H2O

keeping in mind that most formulas don't include the unusual things, like a mole of oxygen will have a very small amount of single oxygen molecules and some ozone, but they don't last long enough to be useful, and they are usually in small enough amounts that scientists use the ever present phrase "small enough to be ignored"