Average Atomic Mass
11 Minutes
Average atomic mass for students explains isotopes, mass spectrometry, and how atomic mass units are calculated. An English science lesson.
- How Dalton and later scientists defined the atomic mass unit
- Choosing carbon-12 and calculating average atomic mass from isotopes
- How a mass spectrometer separates ions using a magnetic field
What this lesson teaches
Average atomic mass is the weighted average mass of all the naturally occurring isotopes of an element. It is calculated by taking into account the individual mass of each isotope and how abundantly it appears in nature.
Defining the atomic mass unit
Early scientists first used hydrogen as a standard of one atomic mass unit, but later shifted to oxygen. Disagreements arose because physicists measured only oxygen-16, while chemists measured an average of oxygen-16, 17, and 18, creating a 0.028 percent difference. To settle this dispute, scientists chose carbon-12, dividing its mass of 1.99 into 10 to the power minus 23 grams by 12 to define one unified atomic mass unit.
Isotopes and average atomic mass
Isotopes are atoms of the same element that have different numbers of neutrons. Carbon has three isotopes: carbon-12 with six neutrons, carbon-13 with seven neutrons, and carbon-14 with eight neutrons. Natural carbon consists of about 99 percent carbon-12 and 1 percent carbon-13, while carbon-14 is extremely rare. Multiplying each isotope's mass by its percentage abundance and adding the values gives carbon an average atomic mass of 12.01 atomic mass units, which is the mass recorded on the periodic table.
How a mass spectrometer works
- A sample of an element is heated until it turns into vapor.
- Electrons are stripped away to convert the neutral atoms into positive ions.
- The positive ions are directed through a magnetic field.
- Isotopes with different masses separate and land in different locations based on their weight.
- The device measures the amount of atoms in each spot to calculate the natural abundance of each isotope.
Subatomic particles and mass
- An atom's mass comes almost entirely from its protons and neutrons.
- A proton and a neutron each have a mass of roughly one atomic mass unit.
- An electron is extremely light, requiring 1,867 electrons to match the mass of just one proton.
- Because electrons contribute so little weight, they are left out of atomic mass calculations.