The empirical formula of a compound shows the simplest whole number ratio of atoms of each element in it. The molecular formula shows the actual number of atoms in one molecule. Ethane has the molecular formula C2H6. The ratio of carbon to hydrogen atoms is 2:6, and dividing both numbers by 2 gives 1:3, so its empirical formula is CH3.
To find an empirical formula from a model or diagram, count the atoms of each element, write them as a ratio and divide by the largest number that goes into all of them. A molecule with 4 carbon atoms and 8 hydrogen atoms is C4H8. The ratio 4:8 simplifies to 1:2, so the empirical formula is CH2. Glucose, C6H12O6, has 6 carbon, 12 hydrogen and 6 oxygen atoms. Dividing by 6 gives 1:2:1, so its empirical formula is CH2O and the molecule has 6 times as many atoms as that formula.
You can also work the other way. If the empirical formula is CH2, a model with 3 carbon atoms must have 6 hydrogen atoms. If the relative formula mass is also given, divide it by the empirical formula mass. CH2 has a mass of 14, so a molecule with Mr = 56 contains 56 ÷ 14 = 4 units of CH2.
Giant ionic compounds, such as sodium chloride, are written only as an empirical formula, because they are not made of separate molecules. NaCl shows that the ratio of sodium ions to chloride ions is 1:1.