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Introduction
In atomic physics and quantum chemistry, the electron configuration is the distribution of electrons of an atom or molecule in atomic or molecular orbital.
According to the laws ofquantum mechanics, an energy is associated with each electron configuration and, upon certain conditions, electrons are able to move from one orbital to another by emission or absorption of a quantumof energy, in the form of a photon.
Knowledge of the electron configuration of different atoms is useful in understanding the structure of the periodic table of elements. The concept is also usefulfor describing the chemical bonds that hold atoms together. In bulk materials this same idea helps explain the peculiar properties of lasers and semiconductors.Description of the evolution of the theories and atomic models

• Aristotle: emphasized that nature consisted of four elements: air, earth, fire, and water.
• Democritus: postulated the existence ofinvisible atoms, characterized only by quantitative properties: size, shape, and motion.
• Dalton: deduced the law of multiple proportions, which stated that when two elements form more than one compoundby combining in more than one proportion by weight, the weight of one element in one of the compounds is in simple, integer ratios to its weights in the other compounds.
• J.J. Thomson: held thatatoms are uniform spheres of positively charged matter in which electrons are embedded.
• Ernest Rutherford: The model described the atom as a tiny, dense, positively charged core called a nucleus, inwhich nearly all the mass is concentrated, around which the light, negative constituents, called electrons, circulate at some distance, much like planets revolving around the Sun.
• Niels Bohr: In1913 he proposed his quantized shell model of the atom to explain how electrons can have stable orbits around the nucleus. The motion of the electrons in the Rutherford model was unstable because,...