Atomic Mass and Atomic Mass Unit (amu)
Defines atomic mass as the mass of an atom relative to 1/12th the mass of a carbon-12 atom, establishing the atomic mass unit (amu).
What is Atomic Mass and Atomic Mass Unit (amu)?
The average mass of atoms of an element, measured on a scale in which a carbon atom (carbon-12) is assigned a mass of exactly 12 units.
Key formula / rule: Definition of amu
Key points
- Define atomic mass and atomic mass unit (amu).
- Explain the basis of the atomic mass scale (carbon-12).
- Differentiate between atomic mass and mass number.
- Understand that atomic masses in the periodic table are weighted averages.
Common exam trap
Confusing atomic mass with mass number (number of protons + neutrons).
Definitions
- Term
Atomic Mass
- Meaning
The average mass of atoms of an element, measured on a scale in which a carbon atom (carbon-12) is assigned a mass of exactly 12 units.
- Term
Atomic Mass Unit (amu)
- Meaning
A unit of mass used to express atomic and molecular masses, equal to one-twelfth the mass of an atom of carbon-12.
- Term
Dalton (Da)
- Meaning
An alternative name for the atomic mass unit (amu).
Learning objectives
Define atomic mass and atomic mass unit (amu).
Explain the basis of the atomic mass scale (carbon-12).
Differentiate between atomic mass and mass number.
Understand that atomic masses in the periodic table are weighted averages.
Relate amu to SI units (kilograms).
Formulae
- Name
Definition of amu
- Note
This defines the standard unit for atomic and molecular masses.
- Expression
1 amu = (Mass of one ¹²C atom) / 12
- Name
Average Atomic Mass
- Note
Used when an element has multiple isotopes.
- Expression
Average Atomic Mass = Σ (Mass of isotopeᵢ × Fractional abundance of isotopeᵢ)
Prerequisites
Basic understanding of atoms and elements.
Concept of isotopes.
Familiarity with the periodic table.
Common mistakes
Confusing atomic mass with mass number (number of protons + neutrons).
Assuming all atoms of an element have the same mass (ignoring isotopes).
Using incorrect conversion factors between amu and kilograms.
Not understanding that the periodic table value is an average.
Keywords
Atomic Mass
Atomic Mass Unit
amu
Dalton
Carbon-12
Isotopes
Relative Mass
Average Atomic Mass
Molar Mass
Practice preview
Which of the following statements accurately describes the atomic mass of an element?…
easy
What is the standard reference used to define the atomic mass unit (amu)?…
easy
The atomic mass of an element is 12.011 amu. What does this value represent?…
medium