Mass
The basic SI unit of mass is the kilogram (kg). In physics, mass is not the same as weight, even though mass is often determined by measuring the object's weight using a spring scale, rather than balance scale comparing it directly with known masses. An object on the Moon would weigh less than it does on Earth because of the lower gravity, but it would still have the same mass. This is because weight is a force, while mass is the property that (along with gravity) determines the strength of this force.
Mass number
The mass number gives an estimate of the isotopic mass measured in atomic mass units (u). For C the isotopic mass is exactly 12, since the atomic mass unit is defined as 1/12 of the mass of C. For other isotopes, the isotopic mass is usually within 0.1 u of the mass number. For example, Cl (17 protons and 18 neutrons) has a mass number of 35 and an isotopic mass of 34.96885. The difference between mass number of an atom and its isotopic mass is known as the mass excess. Mass excess should not be confused with mass defect which is the difference between the mass of an atom and its constituent particles (namely protons, neutrons and electrons).