2.4 - Atomic Mass

Relative Atomic Mass

Used to describe the weighted average of the atomic masses of the isotopes that compose an element as it is found in nature. This term is synonymous with the older term atomic weight, which is still commonly used.

Mass Spectrometry

Measures the masses of individual atoms. Electrons are removed from atoms (or molecules) to produce ions that are accelerated through a magnetic field. The degree of bending in the path of the ions is related to the mass and the charge of the ions. Mass spectrometry is also used to determine the relative amounts of different isotopes in a sample of an element.

Atomic Mass Unit (amu)

One amu is equal to one-twelfth the mass of a carbon-12 atom.

Where is amu located on the periodic table?

The relative atomic mass of each element is listed below its element symbol.

How can you tell if an isotope is more abundant by looking at amu?

You can tell which isotope is more abundant by comparing the element's average atomic mass (the value on the periodic table, measured in amu) to the mass numbers of its naturally occurring isotopes.

The Rule: The average atomic mass of an element will always be closest to the mass of its most abundant isotope.

Is the atomic mass representative of the actual mass of an element?

No

We use ____________ when we refer to an “average atom” of an element, or a group of atoms.

Relative atomic mass