Scientists have identified 112 elements. The smallest unit of each of these pure substances that has its properties is an atom. These units are made of smaller particles. The electron has a negative charge, the proton has a positive charge, and the neutron has no charge. Each substance is given a special symbol of one or two letters. A scientist named Dmitri Mendeleyev created a periodic table. He grouped the substances according to chemical properties. Two or more substances can combine chemically to form a new compound.