Physical Science / Unit 3: Atoms and the Periodic Table
3.2 Atomic Number and Mass Number
Notes
- The atomic number is the number of protons. It identifies the element.
- The mass number is protons plus neutrons.
- Neutrons = mass number - atomic number.
- Example: An atom with atomic number 17 and mass number 35 has 17 protons, 17 electrons (if neutral) and $35 - 17 = 18$ neutrons.
- Changing the number of protons changes the element.
Practice
An atom of sodium has atomic number 11 and mass number 23. How many neutrons does it have?
$23 - 11 = 12$.
What does the atomic number of an element tell you?
The proton count defines the element.
An atom has 20 protons and 20 neutrons. What is its mass number?
$20 + 20 = 40$.
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