Relative Mass Terms

Relative Mass Terms

Key Terms

• Relative atomic mass is the average mass of one atom of the element relative to 1/12 the mass of one atom of carbon-12
• Relative isotopic mass is the mass of an atom of an isotope relatiev to 1/12 the mass of an atom of carbon-12
• Relative formula mass is the average mass of a molecule relative to 1/12 the mass of an atom of carbon-12

Masses of atoms

Since the masses of atoms are too small to be used in chemical calculations, the mass of an atom is expressed relative to a chosen standard of atomic mass. The carbon-12 isotope is taken as a standard reference due to the relative atomic masses being determined by the mass spectrometry and volatile carbon compounds are also widely used in mass spectrometry.

Most elements exist as two or more different isotopes. The mass of an element depends on the relative abundance of all the isotopes present in the sample. In order to overcome this, chemists use an average mass of all the atoms and this is called the relative atomic mass, Ar. This has no units since it's one massed compared with another mass. If we refer to the mass of a…