Structure of the Atom

Atomic Models Everything around us is made up of tiny particles called atoms. There are three subatomic particles namely electron, proton and neutron. Electron (e–1) was discovered by J.J. Thomson. It carries one unit of negative charge. Proton (p+) was discovered by Goldstein. It carries one unit of positive charge. Neutron (n0) was discovered by J. Chadwick. It is a neutral atomic particle. To describe the arrangement of subatomic particles within atom, many models of atom are proposed. Some of them are Thomson’s Model, Rutherford’s Model, Bohr’s Model. Thomson’s Model is also known as the watermelon or the plum pudding model as the electrons in the sphere of the positive charge were just like dry fruits in a spherical Christmas pudding. Rutherford’s alpha-particle scattering experiment led to the discovery of the nucleus of atom. Rutherford’s model of the atom proposed that a very tiny nucleus is present inside the atom and electrons revolve around this nucleus. The stability of the atom could not be explained by this model. In order to overcome the limitations of Rutherford’s model of the atom, Neil Bohr gave a new model for the arrangement of the electrons in the atom. He proposed that electrons are distributed in different shells with discrete energy around the nucleus. If the atomic shells are complete, then the atom will be stable and less reactive.

Some Important Concepts of Atom

The distribution of the electrons into different orbits of an atom was suggested by Bohr and Bury. Shells of an atom are designated as K, L, M, N….. The important terms related to atom are atomic number, mass number, electronic configuration, valency, isotopes and isobars. Atomic Number represents the total number of protons present in the nucleus of an atom. It is denoted by Z. Atomic number is equal to the number of protons or number of electrons Mass number represents the total number of protons and neutrons present in the nucleus of an atom. It is denoted by A. The arrangement of the electrons in the various orbits of an atom of the element is called Electronic configuration. Electrons present in the outermost shell of an atom are called the valence electrons. An outermost shell, which has eight electrons is said to possess an octet. Valency is a combining capacity of an atom. Isotopes are the atoms that have the same atomic number but different mass number. Hydrogen has three isotopes Protium, Deuterium and Tritium. Isotopes have special properties which makes them useful in various fields. Isobars are the atoms that have the same mass numbers but different atomic numbers.

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