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The atoms of a compound are held together by chemical bonds formed either by electron transfer between atoms or by sharing of electrons. According to Kossel's theory, the formation of bond depends upon the number of electrons present in the outermost shell of an atom. Lewis symbol consists of the element symbol surrounded by dots to represent the number of electrons present in its valence shell. According to Octet rule, for an atom to be stable it must possess eight electrons in its outermost shell. A covalent bond is a chemical bond formed by mutual sharing of electron pairs between two atoms. Based on sharing of electron pairs, covalent bond is of three types: single bond, double bond and triple bond. Single covalent bond is formed by the sharing of one pair of electrons between two atoms. Double covalent bond is formed by the sharing of two pairs of electrons between two atoms. Triple covalent bond is formed by the sharing of three pairs of electrons between two atoms. On the basis of polarity, covalent bonds are polar and non-polar. Polar covalent bond is formed between two dissimilar atoms with different electronegativity. Non-polar covalent bond is formed between two similar atoms with same electronegativity. Covalent compounds generally exist as gases or liquids. They have low melting and boiling points and they are poor conductors of heat and electricity. The properties of covalent compounds are different from the properties of electrovalent compounds.

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