Atomic Number, Mass Number and Isotopes – Lesson Summary

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Learning Objective

In this lesson we will learn how atoms are defined by the number of protons they contain, as well as the number of neutrons.

Learning Outcomes

By the end of this lesson you will be able to:

  • Define atomic number.
  • Define mass number.
  • Compare isotopes of an element.

 
atomic number mass number and isotopes lesson contents

(Image: majcot, Adobe Stock)

Lesson Topics

1 | Atomic Number

2 | Mass Number

3 | Isotopes

4 | Summary

Lesson Summary

  • The number of protons in an atom is called the atomic number.
  • Atomic Number  =  Number of Protons
  • The number of protons plus the number of neutrons in an atom is called the mass number.
  • Mass Number  =  Number of Protons  +  Number of Neutrons
  • The atomic number of an element is fixed but the mass number can vary.
  • Atoms that have the same atomic number (number of protons) but different mass numbers (number of neutrons) are called isotopes.

 
 
isotope symbols atomic mass number

The isotope lithium-7 has the atomic number 3 and the mass number 7.

(Header image: Johannes Schneider, Wikimedia Commons)