Atoms are the basic building blocks of all matter.
Matter is anything that has volume and mass. Everything in the physical world that surrounds you is composed of matter. On Earth, matter usually occurs as a solid, a liquid, or a gas. All matter is made of substances called elements. An element is a substance that cannot be broken down into simpler substances by physical or chemical means. For example, gold is still gold whether it is a gold brick, coins, or a statue. Each element has distinct characteristics. You have learned some of the characteristics of the element gold. Although aluminum has different characteristics than gold, both aluminum and gold are elements that are made up of atoms. All atoms consist of even smaller particles— protons, neutrons, and electrons. The center of an atom is called the nucleus (NEW klee us) (plural, nuclei). The nucleus of an atom is made up of protons and neutrons. A proton is a tiny particle that has mass and a positive electric charge. A neutron is a particle with approximately the same mass as a proton, but it is electrically neutral; that is, it has no electric charge. All atomic nuclei have a positive charge because they are composed of protons with positive electric charges and neutrons with no electric charges.
Surrounding the nucleus of an atom are smaller particles called electrons. An electron (e-) has little mass, but it has a negative electric charge that is exactly the same magnitude as the positive charge of a proton. An atom has an equal number of protons and electrons; thus, the electric charge of an electron cancels the positive charge of a proton to produce an atom that has no overall charge. Notice that the electrons in Figure are shown as a cloudlike region surrounding the nucleus. This is because electrons are in constant motion around an atom’s nucleus, and their exact positions at any given moment cannot be determined.