of an electron, proton, neutron is 1/2. The particles having half integral value (1/2, 3/2 ...) of spin are called fermions. The particles having integral value (0,1,2..) of spin are called bosons. Magnetic nature of atoms and molecules The spin quantum number helps to explain the magnetic properties of atoms and molecules. A spinning electron behaves like a micromagnet with a definite magnetic moment. If an atomic or molecular orbital contains two electrons, then their magnetic moments oppose and cancel each other. If all orbitals are doubly occupied by electrons, the net magnetic moment is zero and the substance behaves as diamagnetic; it is repelled by the external magnetic field. If some orbitals are half filled (singly occupied), the substance has a net magnetic moment and is paramagnetic; it is attracted by the external magnetic field. History Early attempts to explain the behavior of electrons in atoms focused on solving the Schrödinger wave equation for the hydrogen atom, the simplest possible case, with a single electron bound to the atomic nucleus. This was successful in explaining many features of atomic spectra. The solutions required each possible state of the electron to be described by three "quantum numbers". These were identified as, respectively, the electron "shell" number , the "orbital" number , and the "orbital angular momentum" number . Angular momentum is a so-called "classical" concept measuring the momentum of a mass in circular motion about a point. The shell numbers start at 1 and increase indefinitely. Each shell of number contains orbitals. Each orbital is characterized by its number , where takes integer values from 0 to , and its angular momentum number , where takes integer values from + to −. By means of a variety of approximations and extensions, physicists were able to extend their work on hydrogen to more complex atoms containing many electrons. Atomic spectra measure radiation absorbed or emitted by electrons "jumping" from one "state" to another, where a state is represented by values of , , and . The so-called "Transition rule" limits what "jumps" are possible. In general, a jump or "transition" is allowed only if all three numbers change in the process. This is because a transition will be able to cause the emission or absorption of electromagnetic radiation only if it involves a change in the electromagnetic dipole of the atom. However, it was recognized in the early years of quantum