In ancient Greek philosophy a word atom was used to describe the smallest bit of matter that could be concived. This "fundamental particle,"to use the present-day term for this concept, was thought of as indestructible; in fact, the Greek word for atom means "not divisible." Knowledge about the size of nature of the atom grew slowly throughout the centruries when people were content merely to speculate about it.
With the advent of experimental science in the sixteenth and seventeenth centuries, progress in atomic theroy qucikened. Chemists soon reconized that all liquids, gases, and solids can be analyzed into their ultimate components or elements. For example, salt was found to be composed of two distinct and diffrent elements, sodium and chlorine, which are joined in an intimate form known as a chemical compound. Air was discovered to consist of a mixture of the gases nitrogen and oxygen. Water was symbolized as HOH, Meaning it consists of two atoms of hyrogen and one atom of oxygen.