What is the difference between atomic weight and atomic mass?

Atomic weight refers to the average mass of atoms of an element, while atomic mass is the mass of a single atom.

Atomic weight, also known as relative atomic mass, is a weighted average that takes into account the abundance of different isotopes of an element. Isotopes are atoms of the same element that have different numbers of neutrons, and therefore different masses. The atomic weight is calculated by multiplying the atomic mass of each isotope by its natural abundance (the percentage of that isotope found in nature), and then adding these values together. This value is dimensionless and is usually expressed on a scale where the atomic weight of carbon-12 is exactly 12.

On the other hand, atomic mass, also known as the atomic mass number, is the total number of protons and neutrons in an atom. It is measured in atomic mass units (amu), where 1 amu is approximately equal to the mass of a single proton or neutron. The atomic mass of an atom can be found by adding the number of protons and neutrons together. For example, a carbon-12 atom has 6 protons and 6 neutrons, so its atomic mass is 12 amu.

In summary, while both atomic weight and atomic mass refer to the mass of an atom, they are used in different contexts. Atomic weight is a weighted average that takes into account the natural abundance of different isotopes, while atomic mass is a count of the total number of protons and neutrons in a single atom.