Each atom has a specific number of positively charged protons and an equal number of negatively charged electrons. In addition, all atoms except the simplest form of hydrogen have electrically neutral particles called neutrons. The protons and neutrons occupy the center of the atom called the nucleus, while the electrons move about in the outer regions of the atom commonly called shells or orbits. The energy of an electron depends on the region in which it moves. Electrons have more energy when they are farther away from the nucleus. The electrons have comparatively little mass. The major portion of the atom's mass is concentrated in the relatively heavy protons and neutrons in the atom's nucleus.
Nineteenth Century chemists were at a disadvantage when they attempted to learn about atoms and their respective elements. J.J. Thomson did not discover the electron until 1897 and the discovery of the proton and neutron did not occur until well into the 20th Century. However, 19th Century chemists weighed the reactants and products of chemical reactions and observed the proportional changes of weight that occurred. From the resulting data, they succeeded in determining the relative atomic weights of atoms. Consequently, they used atomic weight to identify the atoms of each element and it was the atomic weight that Mendeleev used to arrange elements when he constructed his periodic tables.
Elements, such as hydrogen and helium, exist in different forms called isotopes, and each isotope has a different atomic weight. In fact, both hydrogen and helium have an isotope with an atomic weight of three. For this reason, atomic weight is not an exact criterion for identifying an atom as a given element. In contrast, the number of protons in an atom's nucleus unfailingly shows to what element the atom belongs, since every element has atoms with a specific number of nuclear protons and no two elements have the same number. For this reason, elements are now arranged according to the number of protons they possess, technically called their atomic number.
The number of electrons an atom has does not supply exact information concerning the element to which it belongs because atoms can gain or lose electrons and become ions. However, electrons provide exact information in a different way. Electrons occasionally move to a higher energy level. When they return to their original ground state, they emit a photon. The atoms of each element emits its own unique wave length of light and other energy, so that a spectroscopic analysis of the wave lengths emitted reveal the identity of the elements involved. In fact, by spectroscopic analysis, Lockyer discovered the element helium in the Sun
before it was found on Earth.