Relative Atomic Mass, a.k.a. Atomic Weight
Description of Relative Atomic Mass
Relative atomic mass is a dimensionless quantity that represents the average mass of atoms of an element, scaled relative to the mass of carbon-12, which is assigned a value of exactly 12. This measure accounts for the presence of different isotopes of an element and their natural abundance. It provides a standardized way to compare the masses of different atoms.
Atomic Mass Unit (amu)
The atomic mass unit is a standard unit of mass that quantifies mass on an atomic or molecular scale. One amu is defined as one-twelfth of the mass of a carbon-12 atom.
How to Measure Relative Atomic Mass
Measuring relative atomic mass involves several steps:
- Isotope Identification: Determine the different isotopes of the element and their respective masses.
- Natural Abundance: Assess the percentage of each isotope present in a naturally occurring sample.
- Calculation: Multiply the mass of each isotope by its natural abundance and sum the results to obtain the relative atomic mass.
Consider chlorine, which has two stable isotopes: Chlorine-35 and Chlorine-37.
|
Isotope |
Mass (amu) |
Natural Abundance (%) |
|
Chlorine-35 |
34.968853 |
75.76 |
|
Chlorine-37 |
36.965903 |
24.24 |
Relative Atomic Mass = (34.968853 × 0.7576) + (36.965903 ×
0.2424)
Relative Atomic Mass ≈ 26.504 + 8.964
Relative Atomic Mass ≈ 35.468 amu
Relation to Standard Atomic Mass
Standard atomic mass is the weighted average of the masses of an element's isotopes, based on their natural abundance. It is essentially synonymous with relative atomic mass but is often used in precise scientific contexts.
Relation to Mass Number
The mass number is the total number of protons and neutrons in an atom's nucleus. Unlike relative atomic mass, the mass number is always a whole number and specific to each isotope.
|
Measure |
Definition |
Usage |
|
Relative Atomic Mass |
Average mass of atoms relative to carbon-12 |
Comparative analysis in chemistry |
|
Standard Atomic Mass |
Weighted average mass based on isotope abundance |
Precise scientific measurements |
|
Mass Number |
Total number of protons and neutrons in an atom's nucleus |
Identifying specific isotopes |
For more information, please check Stanford Advanced Materials (SAM).
Frequently Asked Questions
What is the difference between relative atomic mass and absolute
atomic mass?
Relative atomic mass is a dimensionless quantity comparing an atom's mass to
carbon-12, while absolute atomic mass is measured in atomic mass units (amu).
How does isotope variation affect relative atomic mass?
Different isotopes have varying masses and natural abundances, which influence
the weighted average that constitutes the relative atomic mass.
Why is carbon-12 used as the standard for relative atomic mass?
Carbon-12 is stable, has a convenient atomic mass, and is widely available,
making it an ideal reference point for atomic mass measurements.
Can relative atomic mass change over time?
Relative atomic mass remains constant for an element under standard conditions,
as the natural abundance of isotopes does not typically change.
How is relative atomic mass used in chemical equations?
It allows chemists to calculate the amounts of reactants and products in moles,
facilitating the balancing of chemical equations.
