Atomic mass

Definition

Atomic mass is the weighted average mass of an atom's naturally occurring isotopes, measured in atomic mass units (amu). It reflects both the mass and relative abundance of each isotope.

5 Must Know Facts For Your Next Test

1. Atomic mass is not a whole number because it is an average of all isotopes' masses.
2. It is measured in atomic mass units (amu), where 1 amu is defined as one-twelfth the mass of a carbon-12 atom.
3. The atomic mass listed on the periodic table accounts for the relative abundances of an element's isotopes.
4. In nuclear physics, understanding atomic mass helps explain nuclear reactions and stability.
5. Isotopic variations impact calculations involving molar masses and material compositions.

Review Questions

• Why is atomic mass generally not a whole number?
• How does the concept of isotopic abundance affect the calculation of atomic mass?
• What unit is used to express atomic mass, and how is it defined?

Related terms

Isotope:

Atoms of the same element with different numbers of neutrons and thus different masses.

Mass Number:

The total number of protons and neutrons in an atom's nucleus.

Atomic Number: The number of protons in an atom's nucleus, defining the element.