Atom the nucleus defines to what chemical element the atom belongs: for example, all copper atoms contain 29 protons. The number of neutrons defines the isotope of the element. The number of electrons influences the magnetic properties of an atom. Atoms can attach to one or more other atoms by chemical bonds to form chemical compounds such as molecules. The ability of atoms to associate and dissociate is responsible for most of the physical changes observed in nature and is the subject of the discipline of chemistry. The idea that matter is made up of discrete units is a very old
Hydrogen atom and spectral frequencies of the hydrogen atom after making a number of simple assumptions in order to correct the failed classical model. The assumptions included: Bohr supposed that the electron's angular momentum is quantized with possible values: formula_4 where formula_5 and formula_6 is Planck constant over formula_7. He also supposed that the centripetal force which keeps the electron in its orbit is provided by the Coulomb force, and that energy is conserved. Bohr derived the energy of each orbit of the hydrogen atom to be: where formula_9 is the electron mass, formula_10 is the electron charge, formula_11 is the vacuum