Pakistan study notes for Matric, Intermediate, B.A, B.COM BSC, M.A, BCS. Free Pakistani Urdu educational school, colleges and University notes.



Bohr suggested the existence of certain circular orbits at definite distance from the nucleus. These orbits are associated with definite energy of the electron increasing outwards from the nucleus. Its evidence is given by the line spectra.

The Bohr’s circular orbits are usually referred to as “Energy levels” or “Shells”.

These are designated as 1, 2, 3, 4 etc. or K. L, M, N. etc. The maximum possible number of electrons a shell ‘n’ can accommodate is given by 2n², i.e. Z, 8, 18, and 32 respectively in the first, second, third and fourth energy level. It may be pointed out that in the outer most shell of any atom, the maximum number of electrons possible is 8 (Except hydrogen whose outer most orbit being first orbit, is to be filled by “2” electrons).


The spectral lines, which correspond to the transition of an electron from one energy level to another, have for the most part, a fine structure, i.e. each line actually consists of several separate, close lying lines as doublets, triplets and so on indicating that some of the electrons of the given energy level have different energies. That is to say, that the electrons belonging to same energy level may differ in their energy. The energy levels are accordingly divided into sub energy levels denoted by the letters s, p, d, f …. for the first four series of lines in the spectra of the alkali metals, based on the terms Sharp, Principal, Diffuse and Fundamental, The number of sub level in a given energy level or shell is equal to its value of ‘n’ for example, with in a principal energy level (n=2) i.e. second energy level, two sub levels are possible which are s and p.


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