Isotopes are copies of atoms but with different numbers of neutrons. Because they have different numbers of neutrons, they have different atomic masses.
An example of a pair of isotopes would be carbon-12 and carbon-14. The mass of an atom is the number of protons and neutrons added together. As carbon has 6 protons, to make carbon-12 (which has a weight of 12) it must also have 6 neutrons. In Carbon 14, it also has 6 protons but must also have 8 neutrons to weigh 14. The following table may make this easier to understand...
Molecular masses are basically the masses of the atoms in a molecule added together. For example a molecule of Hydrogen Gas (H2) is made of two hydrogen atoms. The atomic mass of each hydrogen atom is 1, so the atomic mass of a hydrogen molecule (H2) is 2 (1+1)
Other examples get more complicated as they involve different several different atoms. Some examples are given below using the atomic masses given in the table.
- Water - H2O = (H - 2 x 1) + (O - 1 x 16) = 18
- Ammonia - NH3 = (N - 1 x 14) + (H - 3 x 1) = 17
- Salt - NaCl = (Na - 1 x 23) + (Cl - 1 x 35) = 58
- Ethanol - C2H6O = (C - 2 x 12) + (H - 6 x 1) + (O - 1 x 16) = 46