Elements: Atoms that are the smallest particles of matter. An atom is the smallest particle of an element that has the properties of that element. Salt is an example made up of the elements sodium and chloride. Water is made up of the elements hydrogen and oxygen. Many centuries ago, Greek thinkers arrived at a conclusion that every thing like gold, silver, sulfur, dogs, trees and humans were made up of four different materials called elements. These four elements were earth, air, fire and water. Some other thinkers thought that matter was made up of small particles called atoms. The elements theory became more popular.