He called these particles atomos . In 1803, the British scientist John Dalton improved upon Democritus' idea of atomos and developed the first 'atomic' theory. This theory stated that all matter was composed of tiny particles called atoms . Dalton hypothesized that different types of matter were made of different types of atoms, a simple but revolutionary theory. Dalton knew that there were a limited number of chemically pure substances that were called elements - he hypothesized that these different elements were made of different atoms.
Atoms are indestructible and unchangeable.
Chemical reactions involve the combination of atoms, not the destruction of atoms. All atoms of a given element are identical. Atoms of different elements have different properties. Compounds are formed when atoms of different elements combine chemically. When elements react to form compounds, they react in defined, whole number ratios.