Atoomgewicht

An element's atomic weight, sometimes called atomic mass, is approximately equal to the sum of the number of protons and neutrons. This is expressed in atomic mass units, abbreviated as amu, or Daltons, and is represented near its chemical symbol on the periodic table.

But, most elements have several isotopes that differ in the number of neutrons and so in their masses.

For such elements, the atomic weight is the average mass of all of the isotopes of an element, weighted according to their relative abundance or the percentage of each isotope found in nature.

For instance, chlorine has two naturally occurring isotopes. The mass of chlorine-35 is 34.97 amu and of chlorine-37 is 36.97  amu.

When multiplied by their relative abundances, 75.77% for chlorine-35 and 24.23% for chlorine-37, and added up, the final atomic weight of 35.45 amu is obtained.

Since chlorine-35 is more abundant in nature than chlorine-37, the atomic weight of chlorine is close to 35 amu.