All matter is composed of elements and all elements are composed of atoms.
An atom is the smallest known unit of an element that can exist and still retain all the physical and chemical characteristics of the element. The atom is a combination of particles held together by balanced electromagnetic forces. The fundamental parts of the atom are the nucleus and the electrons. The nucleus is the extremely small core of the atom in which practically all of the atom's mass is concentrated. With the exception of the simple Hydrogen atom, all nuclei are composed of both protons and neutrons.