ATOM is the smallest unit of matter that retains the properties of an element. It consists of a nucleus, made up of protons and neutrons, surrounded by electrons that orbit the nucleus. Atoms combine to form molecules, which are the building blocks of all substances. Each element on the periodic table is defined by its unique type of ATOM. For example, a carbon atom has six protons, while an oxygen atom has eight. The arrangement and number of these atoms determine the chemical properties and behavior of different materials.