Why is the sum of the number of protons and neutrons in an atom not equal to that atom's atomic weight?
The atomic weight is a property that is measured as the sum of the masses of the protons and neutrons of a particular atom. Atoms, however, can have different forms (isotopes) depending on the number of neutrons present. Carbon, for example, exists mostly in nature with 6 neutrons. This isotope is called carbon-12 and has a molar mass of almost 12g exactly. Even so, other isotopes with more neutrons exist, skewing the value of the molar mass. The value is an average, and doesn't predict each isotope independently. Also, the atomic weight of an atom is slightly less than the sum of the masses of their protons and neutrons due to a property known as binding energy mass loss.