The empirical formula of a compound shows the simplest whole number ratio of the atoms of each element in it. It is not always the same as the molecular formula, which shows the actual number of atoms in one molecule.
To work out an empirical formula from the numbers of atoms, divide every number by the largest common factor. Hydrogen peroxide, H2O2, has 2 hydrogen and 2 oxygen atoms. Dividing both by 2 gives the ratio 1 : 1, so the empirical formula is HO. Ethane, C2H6, has a C : H ratio of 2 : 6, which simplifies to 1 : 3, so its empirical formula is CH3. Glucose, C6H12O6, divides by 6 to give CH2O.
You can do the same from a model of a molecule by counting the atoms of each element. A model with 4 carbon atoms and 8 hydrogen atoms (C4H8) gives a ratio of 1 : 2 and an empirical formula of CH2. Going the other way, a formula tells you how many atoms of each element to include in a model: C2H6 needs 2 carbon and 6 hydrogen atoms. If the ratio cannot be simplified, as in water, H2O, the empirical formula is the same as the molecular formula.