An atom is the smallest particle of a chemical element that can take part in a chemical reaction. An atom is usually made up of protons, neutrons, and electrons. Protons and neutrons form the center or nucleus of each atom, whereas the electrons orbit the nucleus in a manner similar to that of planets orbiting the sun in our solar system. The simplest atom is hydrogen, which has a single electron orbiting a nucleus consisting of a single proton; it has no neutron.
Electrons orbit the nucleus, the center point of an atom, in a concentric ring known as a shell. Electrons have a negative electrical charge. Protons and neutrons are clustered into and form the nuclei of all atoms. The electrons orbit around the nucleus in their shells. Protons have a positive electrical charge, whereas neutrons have no electrical charge. The nucleus of an atom makes up 99.9 percent of its mass. This means that most of the weight of any atom is concentrated in its nucleus. The number of protons in the nucleus is the atomic number of any given element. The sum of the neutrons and protons is the atomic mass number. In a balanced or electrically neutral atom, the nucleus is surrounded by as many electrons as there are protons.