Atomic Structure

ELECTRONIC CONFIGURATION

            The specific ways in which electrons are placed in sub shell and orbital is called as electronic configuration.
            To distribute electron in the energy levels, the following three rules are followed:

1.         Paula Exclusion Principle:
            Wolfgang Pauli put forward a principle for filling of orbital.

These two rules can be summarized as that:
“No two electrons in an atom can have the same set of four quantum numbers”.
Explanation:
            Consider a single electron of hydrogen atom, which is placed in 1s orbital. It means that for this electron principle quantum number (n) is 1, Azimuthal quantum number l is 0 and magnetic quantum number ml is 0. The spin quantum number ms for this electron is +1/2. This can be shown by an upward arrow (↑).  Now if another electron is placed in the same “s” orbital then for this electron;
            n = 1, l = 0 and ml = 0, but  it can only be placed if it has opposite spin to that of first electron i.e. -1/2 shown by downward arrow of (↓ ).
Helium atom has 2 electrons in its “s” orbital. It means that the two electron of Helium atom can be placed in n =1 represented by K.
            One electron will have n = 1, l = 0 ml = 0 and ms = +1/2
            Other electron will have n = 1, l = 0 ml = 0 and ms = -1/2

1.  Hund’s Rule:
            The rule states that an electrons repel each other, no electron will join another in a single orbital, if a vacant orbital of the same energy is available. In other words electron will remain unpaired as long as possible.
Example:
            Let us place two electrons in p sub shell. P- sub shell has three orbital which are oriented along the three axis, x, y and z. they are designated as Px, Py, and Pz. To obey Hund’s rule the two electron will occupy separate orbital in sub-shell P and will not pair up ,as sufficient orbital are available to place them in unpaired form. Thus

            Electron occupies orbital in such a way so as to minimize the energy of the atom. The electrons are filled in sub-shell according to their increasing energy. Electron are first put in is, then 2s, 2p, 3s. ….and so on; the filling of various orbital in shown: