Atom is the fundamental building block of all stuff, or what scientists like to call "matter". An individual atom is very small. In fact, the smallest type of atom, hydrogen, has a diameter of 10-8 cm. Every single object is composed of atoms. Our body is made up of many, many individual atoms. There are also many different types of atom. These different types are called elements. Examples of some elements are hydrogen, oxygen, and helium. Under normal conditions many atoms can stick together to form larger, different stuff. Scientists call material that results from the joining of different types of atoms "compounds". Atoms are not the smallest things there but are made up of still smaller stuff. Also, atoms can be broken apart into smaller constituents. An early Greek who postulated the existence of atoms Democritus (460-357 BCE), who said the matter is made of different kinds of invisible atoms existing in otherwise empty space. Democritus made no attempt to think of a way to determine whether atoms exist or not. Evidently, the idea of atoms just appealed to him philosophically. The idea that matter could be infinitely subdivided appealed to other Greek philosophers, among them Aristotle, seldom made any attempt to verify their hypotheses. Although Aristotle emphasized observation, he did not restrict his theoretical ideas to object he could observe. Aristotle was a great philosopher but his science left much to be desired. Many theories go beyond observation In the 1800's an English chemist, John Dalton performed experiments with various chemicals that showed that matter, indeed, seem to consist of elementary lumpy particles (atoms). Although he did not know about their structure, he knew that the evidence pointed to something fundamental.
It was discovered that many substances, called “compounds,” are made of simpler substance, called “elements,” which combine in definite proportions. The smallest amount of a compound is called a molecule,