Slide 2 of 11
Notes:
Nowadays we understand that an atom consists of a heavy, positively charged nucleus surrounded by lighter electrons that form a kind of “cloud” around the nucleus.
In the illustrations at left, the size of the nucleus has been exaggerated about 100,000 times to reveal the structure of the nucleus, which contains most of the mass of the atom. Think of the little black balls as neutrons and the little red ones as protons.
Nuclei can be though to as made of an integer number of baryons (protons and nuclei) each having a mass of about 1 atomic mass unit (u). That explains why the masses of nuclear isotopes are usually close to an integer number of atomic mass units.
In effect, Avagadro’s number is the ratio between the (rather arbitrary) macroscopic unit of mass (gram or kg) and the average mass of a baryon in the Carbon-12 nucleus.
