Atoms
Atoms come in about 100 varieties called elements. All the matter in the universe
is made up of these elements in combination with one another. You have probably
heard the names of some elements including: carbon, hydrogen, oxygen, nitrogen,
gold, silver, copper, and neon.
Often, atoms bond together to make molecules. Individual molecules are so small they can only be seen with scanning probe microscopes.
An element has properties that can change depending on whether or not it is combined with other elements. For example, oxygen and hydrogen are both gases at room temperature. But one oxygen atom combined with two hydrogen atoms makes one water molecule. Water is a liquid at room temperature. A drop of water contains millions of millions of water molecules.