Stoichiometry is the part of chemistry that studies the proportions of elements that combine or react.
Atomic Mass (u)
It is the mass of the atom measured in units of atomic mass (u). Atomic mass indicates how many times the atom considered is heavier than of isotope 12Ç.
In nature, almost all elements are mixtures of their isotopes with different mass percentages. These percentages are called relative abundances. See the relative abundance of chlorine:
The atomic mass of chlorine that appears in the periodic table of elements is the weighted average of these masses. The calculation is done as follows:
See the percentage of hydrogen isotopes in nature:
In the past, the term "atomic weight" was used, but this term should be avoided. To determine the atomic masses of the elements a device called mass spectrometer is used.