An atom is the smallest unit of matter that defines a chemical element. It is a fundamental piece of matter that cannot be divided without the release of electrically charged particles. Atoms are composed of a central nucleus made up of protons and neutrons, which are surrounded by one or more negatively charged electrons. The number of protons in the nucleus, known as the atomic number, primarily determines where that atom fits on the periodic table. The number of protons in the nucleus also defines in large part the characteristics of an atom. Electrons contribute only a tiny part to the mass of the atomic structure, however, they play an important role in the chemical reactions that create molecules. The mass of an atom is measured in atomic mass units, and the atomic weight is the total mass of an atom, including the protons, neutrons, and electrons. All atoms are roughly the same size, whether they have 3 or 90 electrons, and the radius of an atom measures 1-2 angstroms. Atoms are extremely small, typically around 100 picometers across, and humans cannot see them with conventional microscopes.