Same element, different mass
Isotopes are atoms of the same element with the same number of protons but different numbers of neutrons. Because the proton count stays the same, the atoms remain the same element; because the neutron count changes, their masses differ.
A mass number describes one isotope by counting its protons and neutrons. Most periodic tables show a decimal called the standard atomic weight. It combines isotope masses according to how common those isotopes are in representative samples. For an element without a standard atomic weight, some tables instead show one isotope’s mass number in brackets.
Weight each isotope by how common it is
Convert each percentage abundance to a decimal, multiply it by that isotope’s mass, and add the products. An isotope that is more common contributes more to the final average.
The answer should fall within the isotope-mass range. In a two-isotope example, it lies closer to the more abundant isotope; with three or more, consider all weighted contributions. No individual atom has to equal that average; the value summarizes a mixture.
Remember these ideas
Key ideas
- Isotopes of an element have the same proton count and different neutron counts.
- Average atomic mass is a weighted average, not the mass number of one atom.
- Percentage abundance must be converted to a fraction or decimal before multiplying.
- A quick range and closeness check can catch arithmetic mistakes.
Make the reasoning visible
Find an average for two isotopes
- 1
Suppose 25% of a sample has isotope mass 10 u and 75% has isotope mass 11 u.
- 2
Convert the abundances: 25% = 0.25 and 75% = 0.75.
- 3
Weight and add: (0.25 × 10 u) + (0.75 × 11 u) = 2.50 u + 8.25 u = 10.75 u.
- 4
Check: 10.75 u is between 10 u and 11 u and is closer to 11 u, the more abundant isotope.
Try the next step
Calculate and check
An element has two isotopes: 60% has a mass of 24 u and 40% has a mass of 26 u. What is the average atomic mass?
How could you predict which isotope mass the average will be closer to before calculating?