An atomic mass unit is defined as being equal to 1/12th the mass of an atom of carbon-12 and is approximately 1.66x10-27 kilograms. The atomic mass unit is the unit of measure used when calculating the atomic mass of an element.
The atomic number of an element is defined by the number of protons in its nucleus. Its mass is determined by the number of protons plus the number of neutrons in the nucleus. Different species of the same atomic element will have different mass numbers because the number of neutrons in the nucleus is different. Each of the species of an element is called an isotope.
Atomic mass is an average mass
To calculate the atomic mass of an element, the mass of all of its isotopes have to be taken into account. The atomic mass is therefore an average mass. To arrive at this average mass, the percentages of the known isotopes of the element with their individual masses have to be summed up and averaged out. The average atomic mass of an element as then expressed in atomic mass units. This is why the atomic mass of chlorine is 35.5.