Atomic number The number of elementary positive charges (protons) contained within the nucleus of an atom. It is denoted by the letter Z. Correspondingly, it is also the number of planetary electrons in the neutral atom. The concept of atomic number emerged from the work of G. Moseley, done in 1913-1914. He measured the wavelengths
Atomic nucleus The central region of an atom. Atoms are composed of negatively charged electrons, positively charged protons, and electrically neutral neutrons. The protons and neutrons (collectively known as nucleons) are located in a small central region known as the nucleus. The electrons move in orbits which are large in comparison with the dimensions of
Atomic mass unit An arbitrarily defined unit in terms of which the masses of individual atoms are expressed. One atomic mass unit is defined as exactly 1fi2 of the mass of an atom of the nuclide 12C, the predominant isotope of carbon. The unit, also known as the dalton, is often abbreviated amu, and is