Atoms are the basic units of matter and the defining structure of elements. The term “atom” comes from the Greek word for indivisible, because it was once thought that atoms were the smallest things in the universe and could not be divided. The structure of an atom is a central nucleus composed of protons and neutrons with electrons orbiting around this nucleus.
All matter is made up of atoms, which are in turn made up of smaller particles: protons, neutrons and electrons. Protons have a positive charge, electrons have a negative charge, and neutrons have no electrical charge. Electrons orbit the nucleus in shells. The number of protons in an atom’s nucleus determines what element it is: hydrogen has one proton, helium has two, lithium has three and so on through the periodic table to uranium, which has 92.
The size of an atom is about 10-8 cm—much smaller than anything we can see with our eyes or even a microscope. In fact, most of an atom is empty space! If we could magnify an atom to the size of a tennis ball, its nucleus would be only about as big as a pea at its center. Even though they are very small, atoms are extremely important because they make up everything around us: air, water, furniture…everything!