An atom is the smallest particle into which an element can be resolved that is unique to that element. An atom consists of a nucleus and one or more electrons that move about the nucleus. The nucleus lies at the center of the atom and consists of two types of particles, protons and neutrons. Almost all the mass of an atom is in the nucleus. Protons bear a positive electric charge.
Electrons carry a negative charge, equal in magnitude and opposite to the charge of a proton.
Protons and neutrons are nearly the same size and weight. Atoms are extremely small, on the order of a few ten billionths of a meter. An atom as a whole is electrically neutral. The positive nuclear core is surrounded by a negative cloud of electrons. A neutral atom has equal numbers of electrons and protons.