An atom is comprised of a central portion called a nucleus that contains protons and neutrons. Electrons orbit the nucleus and are typically found in a one-to-one ratio with protons. Different stable atomic configurations are called elements. Hydrogen is unique in having one proton, one electron and no neutrons.
The mass of an atom is primarily determined by the number of protons and neutrons it has, as electrons have very little mass. Protons have a positive energy charge, electrons have a negative energy charge, and neutrons have no charge. The mass of an atom of a particular element may vary, as some atoms of the same element have different quantities of neutrons. These atoms are called isotopes.
Atomic structure directly relates to an element’s physical and chemical properties. Every atom tries to have a full outer electron orbital, and those that don’t have full orbitals take part in chemical reactions that can form new bonds. Atoms that do have full outer shells, such as the noble gases, are not reactive.
The Russian chemist Dmitri Mendeleev proposed arranging atoms by their atomic mass, now known as the periodic table of elements. This arrangement is still used today and is updated whenever a new element is proven to exist. Only 92 elements occur naturally, as of 2014. The most common are hydrogen, carbon, nitrogen and oxygen.