The earliest ideas about atoms, which were developed by John Dalton, stated that 1) all the matter is composed of atoms; 2) atoms were indivisible tiny particles; 3) all the atoms of an element were identical, with the same size, same masses and same chemical properties; 4) the atoms of different elements were different; 5) different atoms combine is simple ratios of whole numbers to form compounds; 6) in a chemical reaction, the atoms separate or recombine to form new compounds. Nowadays, as result of the the research, most of those ideas have been modified: 1) still the matter is constituted by atoms; 2) atoms are formed by smaller particles (electrons, protons, neutrons) and they can be split; 3) isotopes are atoms of a same element that differ in the number of neutrons; 4) still atoms of different elements are different; 5) and 6) are still valid.