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Atomic Structure

Introduction

A smallest unit of element which consists of a dense, central positively charged nucleus surrounded by negatively charged electrons is called as atom. Atom contains electrons, protons and neutrons. Hydrogen is the lightest of all elements and the nucleus of hydrogen atom contains one proton surrounded by one electron.

Rutherford’s theory of atom proposed a model for hydrogen atom in which electrons were moving in a circular orbit around the protons in a nucleus. However, this model was not accepted because of its lack of stability. Latter, Niels Bohr has proposed a model on quantum theory of radiation, which is mainly based on hydrogen atom. According to Bohr’s model, hydrogen atom contains a nucleus having only one proton and one electron moving around the nucleus in a circular path known as orbit. Energy level of electron in an atom is indicated by integers called quantum number. In hydrogen atom, the permissible energy level is described by four quantum numbers – principal quantum number, angular momentum quantum number, magnetic quantum number and spin quantum number.

According to general law of nature, a physical system has maximum stability in its lowest energy state. An electron in an atom first enters into lowest energy orbital and then enters into high energy orbital step-wise. This type of arrangement of electrons is known as Auf-bau principle. Hund’s rule explains that electrons occupy orbital of equal energy such that maximum possible number of electrons remains unpaired and their spins remain parallel. However, Pauli’s principle is based on the fact that no two electrons in an atom can have the same set of four quantum numbers.

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