Physical Science / Unit 3: Atoms and the Periodic Table
3.3 Isotopes and Atomic Mass
Notes
Isotopes are atoms of the same element with different numbers of neutrons, so different mass numbers.
- Carbon-12 has 6 protons and 6 neutrons. Carbon-14 has 6 protons and 8 neutrons.
- Isotopes have the same chemical behavior because they have the same number of electrons.
- Atomic mass on the periodic table is a weighted average of an element's isotopes.
- Example: Chlorine is 75% Cl-35 and 25% Cl-37. Average $= 0.75(35) + 0.25(37) = 26.25 + 9.25 = 35.5$ amu.
Practice
How does carbon-14 differ from carbon-12?
Both have 6 protons; carbon-14 has 8 neutrons versus 6.
Isotopes of an element always have the same number of:
The proton number defines the element.
An element has two isotopes: 50% of mass number 10 and 50% of mass number 12. What is its average atomic mass?
$0.50(10) + 0.50(12) = 11$.
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