Fundamentals of Atomic Energy

Essentially, an atom in a miniature solar system in which negatively charged electrons rotate in orbits around a positively charged nucleus. The total negative charge of the electrons is equal to the total positive charge of the nucleus, so that the atom as a whole exhibits zero charge. The nucleus consists of elementary particles called protons and neutrons; the protons carry the positive charge and the neutrons are neutral. Protons and neutrons have almost the same mass, which is 1,840 times the mass of an electron; the number of protons and neutrons in the nucleus thus determine almost completely the atomic weight of the element.