Isotopes and Atomic Mass Calculations

Overview

This lesson breaks down what isotopes are and shows how to calculate average atomic mass and percent abundance, with clear step-by-step examples.

Isotopes: Key Concepts

• Isotopes are just different versions of the same element—they have the same number of protons but a different number of neutrons.
• All isotopes of an element have the same atomic number, but their masses are different.
• The atomic mass you see on the periodic table is actually the average of all the isotopes of that element.

Calculating Average Atomic Mass

• Percent abundance is the percentage of each isotope found in nature.
• To find the average atomic mass:
  1. Change each percent abundance into a decimal (divide by 100 or just move the decimal two places left).
  2. Multiply each isotope’s mass by its decimal abundance.
  3. Add up all those results to get the average atomic mass.
• Round your final answer to match the number of significant figures in the data you started with.

Example 1: Average Atomic Mass Calculation

• Four isotopes of element X are given, each with a mass and percent abundance.
• Convert each percent to a decimal and multiply by its mass.
• Add them all up to get the average atomic mass: 87.62 amu (rounded to four significant figures).

Calculating Percent Abundance

• If you know the average atomic mass and the masses of two isotopes, you can find their percent abundances:
  • Let x = decimal abundance of isotope 1, and (1 – x) = decimal abundance of isotope 2.
  • Set up the equation:
    Atomic mass = (x × mass₁) + [(1 – x) × mass₂]
• Solve for x, then subtract from 1 to get the other isotope’s abundance.
• Change the decimals to percentages by multiplying by 100.

Example 2: Percent Abundance Calculation

• For copper:
  • Cu-63: 69.15%
  • Cu-65: 30.85%
• These percentages show how much of each isotope is in natural copper.

Key Terms & Definitions

• Isotope: An atom with the same number of protons but a different number of neutrons.
• Percent Abundance: The percentage of a certain isotope in a sample.
• Average Atomic Mass: The weighted average mass of all the isotopes of an element.
• Significant Figures (Sig Figs): The digits in a measurement that are certain, plus one that’s estimated.

Action Items / Next Steps

• Try more practice problems on percent abundance and average atomic mass to get ready for exams.
• Go over your textbook or class notes about isotopes and atomic mass calculations.