Some ancient Greeks noticed that substances such as iron, gold, lead, silver were 'pure' and could not be broken down into simpler substances by heat. Wood, cloth, plants, and stone, however, were complex materials that could be destroyed by heat or fire and could not be reformed.

Much later in history, scientists recognized that the pure substances seen by the Greeks were 'un-cuttable' or 'without - cut' ('a-tamos' in Greek). Atoms, it turned out after a lot of research, were pure, elementary, building blocks from which all more complex, compound, materials were constructed.

Today we call the pure substances, elements the building blocks atoms and the combinations of two or more different atoms, compounds.

In the 1820's in England John Dalton first coined the word atom. He also suggested that all the atoms in an element were identical, but that atoms in different elements were somehow different to one another. This idea, that there could be different atoms with different properties started the sciences of Chemistry and Atomic Physics.

It looked as if the chemical and physical properties of substances could all be explained by the internal properties of the atoms within them.

Since the time of Dalton, great progress has been made in the understanding of atomic structure. We now know a lot about the particles and forces holding an atom together - in fact you can get College degrees in subatomic structure. But, simplifying a lot, there are three stable subatomic particles, which, in various combinations, form all atoms.

Protons positively charged particles with a mass of one unit (known as an amu, or atomic mass unit).
Neutrons neutral particles (no charges) also with a mass of one unit.
Collectively, protons and neutrons are known as nucleons.
Electrons negatively charged particles about 1/2000 the mass of a proton or neutron.