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In the hydrogen atom, the single electron revolves round the nucleus
in any orbit extended up to infi nity, and the path is circular in nature. This
model of atom has corroboration from spectroscopic observation. How-
ever, on the demand of fi ner resolution spectroscopic observation, the
hydrogen atomic model was modifi ed by Sommerfeld who replaced the
circular orbit by elliptical path and incorporated the relativistic correction
of electron mass.
10.1.4 SHELL MODEL
In atomic physics and quantum chemistry, electron configuration is the ar-
rangement of electrons of an atom, a molecule, or other physical structure.
It concerns the way electrons can be distributed in the orbitals of the given
system (atomic or molecular, for instance). The shell model or electron
configuration that was developed from the classical concept of quantum
numbers and Pauli exclusion principle has been manifest in the quantum
mechanical model of the atoms.
Like other elementary particles, the electron is subject to the laws of
quantum mechanics, and exhibits both particle-like and wave-like nature.
Formally, the quantum state of a particular electron is defi ned by its wave
function, a complex-valued function of space and time. According to the
Copenhagen interpretation of quantum mechanics, the position of a par-
ticular electron is not well defi ned, and the concept of probability picture
was invoked in atomic structure problem.
Energy is associated with each electron confi guration, and the knowl-
edge of the electron confi guration of different atoms is useful in under-
standing the structure of the elements of the periodic table. The concept
is also useful for describing the chemical bonds that hold atoms together.
The quantum mechanical study of many electron atoms is the Hartree-
Fock (HF) model or its numerous variants. However, Schrodinger equa-
tion for hydrogen atom was solved and showed that electrons may occur in
different quantized energy level but do not rotate in a circular path of Bohr
model. Hartree-Fock model also calculates the same shell model of atom-
ic structure. The shell structure is revealed in X-ray studies [1]. Therefore,
the identical conclusion was made both by the quantum mechanics and
classical mechanics that there are shells and subshells in atoms and mol-
ecules shell model and was verifi ed by X-ray (experimental) method [1].
 
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