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After reading this section you will be able to do the following:

  • Give a basic definition of matter.
  • Describe a molecule.

The Atom

All matter such as solids, liquids, and gases, is composed of atoms. Therefore, the atom is considered to be the basic building block of matter. However, atoms are almost always grouped together with other atoms to form what is called a molecule. Only a few gases such as helium are composed of individual atoms as the structural unit.


Atoms are extremely small. The radius of a typical atom is on the order of 0.00000000001 meter and cannot be studied without very powerful microscopes. In the image below you can see how a scanning electron microscope can be used to magnify things until very small details appear relatively big.


In the next two sub-units you will learn about what a material composed of only one type of atom is called and you will look at the basic model of an atom.


  1. All matter is composed of atoms.
  2. A molecule is a group of atoms bunched together.