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Ionic-bond model explain the formation of positive and negative ions in terms of electron transfer from an atom having a low electronegativity to one having a high electronegativity. Type of chemical bonds depends on difference in electronegativity between atoms. We shall consider a covalent bond in detail below. The valence electrons are the electrons in the outermost shell on an atom that can be gained or lost in a chemical reaction. He proposed that chemical bonds form between electrons residing in the outermost, or valence, shell of each bonding atom. Lewis called an electron-pair bond between atoms a covalent bond. In 1916, an American scientist Gilbert Lewis (1875-1946) suggested that atoms in molecules might be held together by an electrostatic force attraction between atomic nuclei and electron-pairs shared by the bonding atoms. Ionic crystals have relatively high melting and boiling points, are brittle and are easily broken when a stress is exerted on them.
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They conduct electric current in the liquid phase but not in the solid phase. A large number of substances classified as ionic salts exists. Where k is constant of proportionality, q 1 and q 2 are the charges of the ions and r is interatomic distance. The force of attraction between ions may be expressed by the inverse square law The electrostatic force of attraction between the oppositely charged ions is called ionic bonds. A condition for the formation of a chemical bond is a decrease in the potential energy of a system of interacting atoms. The bond dissociation energy is always positive because otherwise the chemical bond would spontaneously break with the liberation of energy. The bond dissociation energy, also called simply the bond energy, is a measure of its strength. Since nuclei of atoms bear positive charges, we can say that all chemical bonds are formed as the result of simultaneous attraction of two or more nuclei for electrons. The electrostatic force of attraction which hold atoms, ions and molecules together, are, in general, referred to as chemical bonds. When atoms interact, a chemical bond may appear between them that leads to the formation of a stable polyatomic system- a molecule, molecular ion, or crystal.