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Atomic number, ions, and isotopes

Atomic number, ions, and isotopes

Author: Jennifer Roushar
  1. Define atomic number and explain what happens if the number of protons in an atom changes.

  2. Define an ion and explain what happens if the number of electrons in an atom changes.

  3. Define an isotope and explain what happens if the number of neutrons in an atom changes.

This packet will help a learner seeking to understand variations in atomic composition.

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Review of Atomic Structure

Atoms are made up of three particles- protons, neutrons, and electrons. 

1.  Protons have a positive charge and are found in the nucleus.

2.  Neutrons have no charge and are also found in the nucleus. 

3.  Electrons are negative and are found outside the nucleus in regions called orbitals. 

When the number of protons is equal to the number of electrons, then an atom is said to be neutral, meaning no charge.  Electrons have such a small mass compared to protons and neutrons that they do not contribute to the atomic mass of a substance.

Source: Jennifer Roushar

Atomic Number of an Atom


Source: Jennifer Roushar; Images from

What is an isotope?


Source: Jennifer Roushar; Images from;;;

Isotope Simulations

If you are having a hard time understanding what an isotope is, go to the following website:


Click on Run Now.  You can then click on an element and add/subtract neutrons.  This well then give you information about the isotope- such as its symbol, abundance, structure and whether or not it is stable.

What is an Ion?


Source: Jennifer Roushar; Images from