This quantum number refers to the two possible orientations of the spin of an electron, one clockwise and the other anticlockwise spin.
    These are represented by +1/2 and -1/2. If both are positive values, then the spins are parallel otherwise the spins are anti-parallel. The importance of the spin quantum number is seen when electrons occupy specific orbitals in multi-electron atoms.

  • How do electrons in an atom occupy shells, sub-shells and orbitals?

    The distribution of electrons in shells, sub-shells and orbital in an atom is known as electronic configuration.

    Electronic Configuration
        Let us first consider the hydrogen atom for understanding the arrangement of electrons, as it contains only one electron.
    The shorthand notation consists of the principal energy level (n value), the letter representing sub-level (l value), and the number of electrons (x) in the sub-shell is written as a superscript as shown below:

    nl x

    For the hydrogen (H) atom having atomic number (Z) = 1, the number of electrons is one, then the electronic configuration is 1s1 .

        The electron configuration can also be represented by showing the spin of the electron.
            For the electron in H, as you have seen, the set of quantum numbers is: n = 1, l = 0, ml = 0, ms = ½ or – ½.
             For many-electron atoms, we must know the electron configuration of the atom. The distribution of electrons in various atomic orbitals provides an understanding of the electronic behavior of the atom and, in turn, its reactivity. Let us consider the helium (He) atom.

    To describe the electronic configuration for more than one electron in the atom, we need to know three principles:
        Those are the Pauli Exclusion Principle, Aufbau principle and Hund’s Rule.
    Let us discuss them briefly.



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