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

  • Define an atom.
  • Define a Basic Element.

All matter, such as solids, liquids and gases, is composed of atoms. Any material that is composed of only one type of atom is called a chemical element, a basic element, or just an element. An atom is the smallest particle of any element that still retains the characteristics of that element. A piece of an element that we are able to see or handle is made of many, many atoms and all atoms are the same...they all have the same number of protons.   Protons and other subatomic particles will be discussed a little later.  The atoms of different elements are different from each other because they have different numbers of protons. The graphic below illustrates this point by showing the atoms of two elements in the containers of oxygen and hydrogen.

The atoms in oxygen are identical to each other. The atoms in hydrogen are identical to each other. However, the atoms of oxygen are different from the atoms of hydrogen.

Compounds, like water, are formed by combining the atoms of different elements together according to some chemical formula.


  1. An atom is the smallest particle of an element that remain identical to all other particles.
  2. The atoms of one element are different from those of all other element.
  3. Compounds are made when atoms of different elements are chemically combined together.