Isotopes — Science definition
Atoms of the same element (same number of protons) that have different numbers of neutrons, and therefore different mass numbers.
Where it is taught
- Isotopes & Average Atomic Mass — Atoms, Electrons & Periodicity, Physical Science: Chemistry, Grade 11
In use
Isotopes are atoms that share the same number of protons — so they are the same element — but carry different numbers of neutrons, and therefore different mass numbers.
Isotopes & Average Atomic Mass
Questions using isotopes
Problem 1
Two isotopes of the same element must have the same number of ___ but different numbers of ___.
- neutrons; protons
- protons; neutrons
- electrons; protons
- protons; electrons
Answer and explanation
protons; neutrons
Isotopes share the proton count (same element) but differ in neutrons, which changes the mass number.
Problem 2
An element has two isotopes: one of mass 20.0 amu at 90.0% abundance and one of mass 22.0 amu at 10.0% abundance. Find the average atomic mass in amu.
Answer and explanation
20.2 amu
Average = 20.0 × 0.900 + 22.0 × 0.100 = 18.0 + 2.2 = 20.2 amu.
Problem 3
Copper has two isotopes: copper-63 (62.93 amu, 69.15%) and copper-65 (64.93 amu, 30.85%). Find copper's average atomic mass in amu.
Answer and explanation
63.55 amu
62.93 × 0.6915 = 43.52 amu; 64.93 × 0.3085 = 20.03 amu; sum = 63.55 amu.
Problem 4
Chlorine's average atomic mass, 35.45 amu, is much closer to 35 than to 37. What does that tell you about the abundances of its two isotopes?
Answer and explanation
Chlorine-35 must be far more abundant than chlorine-37 (roughly 75% versus 25%), because a weighted average is pulled toward the mass of the most common isotope.