Atomic Structure

1. Introduction

The present chapter does not provide much of a historical perspective of how the internal structure of the atom was discouered, rather it deals with the dominant wave character of electrons, their movement inside an atom,how to write electronic configuration, what are the rules required for writing electronic configuration and the maximum probability zones of finding electrons also referred as orbitals etc. However, it is relevant to mention
that all Atoms are made from three subatomic particles Protons, Neutron and Electrons. The important points to keep in mind are as follows:
1. Protons and Neutrons have almost the same mass, while the electron is approximately 2000 times
lighter.
2.Protons and Electrons cary charges of equal magnitude, but opposite charge. Neutrons cary no charge
(they are neutral).

it was once thought that protons, neutrons and electrons were spread out in a rather uniform fashion to torm the atom but now it has been clarified that the actual structure of the atom is quite different. Appearance wise in an atom, protons and neutrons are held together rather closely in the center of the atom. Together they make up the nucleus, which accounts for nearly all of the mass of the atom. Electrons move rapidly around the nucleus and constitute almost the entire volume of the atom. Although quantum mechanics are necessary to explain the motion of an electron about the nucleus , we can say that the distribution of electrons about an atom is such that the atom has a spherical shape. Atoms have sizes on the order of 1-5 angstrom and masses on the order of 1-300 a.m.u. To put the mass and dimensions of an atom into perspectve consider the following analogies. If an atom were the size of a footbaill stadium, the nucleus would onlybe the size of a small marble.

What holds an atom together has a fairly easy explanation. The negatively charged electron is attracted to the
positively charged nucleus by Coulombic forces of attraction. The protons and neutrons are held together in the nucleus by the strong nuclear force. The structure of the atom relate to its properties. It is because chemical reactions involve either the transfer or the sharing of electrons between atoms. Therefore, the chemical reactivity is primarily dependent upon the number of electrons in an atom of that element. Protons also play a significant role because the tendency for an atom to either lose, gain or share electrons are dependent upon the
charge of the nucleus. Therefore, one can say that the chemical reactivity of an atom is dependent upon the number of electrons and protons, and independent of the number of neutrons. The mass and radioactive properties ofan atom are dependent upon the number of protons and neutrons in the nucleus. Modem view of alomic sirucure suggests that electons have pronounced wave character and certain wave functions are used to express their movement inside an atom.