It will never be possible to weigh a single atom or molecule. However, some experiments allow us to measure relative masses. Chemists therefore defined the atomic mass unit as follows: a carbon-12 atom which consists of six protons, six neutrons and six electrons weighs exactly twelve atomic mass units. The weight of an atomic mass unit remains unknown. However, it is known that twelve atomic mass units have the same mass as one carbon-12 atom. The experiment made it possible to establish the relative weights of the isotopes. For example, a carbon-13 atom weighs 1.08361 times more than a carbon-12 atom ; its weight is therefore 13.00335 atomic mass units. It was also determined that 98.89% of carbon atoms are carbon-12 and 1.11% are carbon-13. By calculating the weighted average of the two isotopes, we obtain the mass of the average carbon atom, or 12.01 atomic mass units . Please note that this is not the actual mass of a carbon atom. By analogy, the average size of a Canadian family is 3.9 people, but no family actually has 3.9 members; This is just an average. Avogadro's number was also set arbitrarily. Just as twelve objects are worth a dozen, Avogadro's number is worth a mole. The exact value of Avogadro's number changes over time, but the definition remains the same. This is the number of atoms contained in exactly twelve grams of pure carbon-12 . Currently, the accepted value for Avogadro's number is 6.02214129 x 1023, but it is sufficient to use 6.022 x 1023 for this course. These two definitions allow us to understand the relationship between the average mass of an atom and the mass of a mole of atoms : an average carbon atom weighs 12.01 atomic mass units while a mole of carbon atoms weighs 12.01 grams. Using these definitions, the mass of an individual molecule or atom is calculated by dividing the molar mass by Avogadro's number. The mass of a molecule is also calculated by adding the atomic masses. As mentioned in the introduction, even if the mass of a molecule or atom can be calculated, it will never be possible to measure it.