Atomic Mass
Definition and meaning of Atomic Mass in chemistry.
Atomic mass is the weighted average mass of all naturally occurring isotopes of an element, expressed in atomic mass units (amu). It accounts for both the mass and relative abundance of each isotope.
In more detail
While each isotope of an element has a whole-number mass number, the atomic mass shown on the periodic table is a decimal value because it is calculated as a weighted average. For example, if an element has two isotopes with different abundances, the atomic mass falls between their individual masses. This distinction is crucial for accurate stoichiometry and molecular weight calculations in chemistry, since chemical reactions use atomic masses, not mass numbers.
Key facts
| Unit of Measurement | Atomic mass unit (amu) |
|---|---|
| Type of Average | Weighted average of isotope masses |
| Common Symbol | Decimal number (e.g., 12.011) |
| Field | General Chemistry |
Carbon's two main stable isotopes are carbon-12 (98.89% abundant, exactly 12 amu) and carbon-13 (1.11% abundant, 13.003 amu). The atomic mass of carbon is approximately 12.011 amu, calculated as: (0.9889 × 12) + (0.0111 × 13.003) ≈ 12.011 amu.
Frequently asked questions
What's the difference between atomic mass and mass number?
Mass number is a whole number (sum of protons and neutrons), while atomic mass is a decimal weighted average accounting for the natural abundance of different isotopes.
Where do I find atomic mass values?
Atomic mass is listed on the periodic table for each element, typically displayed below the element symbol as a decimal value.