In the Electron dot structural presentation, dots or crosses represent valence electrons which are put around an atom.
Covalency of an atom is equal to the number of covalent bonds formed by the atom.
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(a) When two atoms share one pair of electrons, by contributing one electron from each of the atoms, a single covalent bond is formed.
Example – Formation of H2
Hydrogen atom has one electron in its K shell and it is represented by one cross around hydrogen atom. It requires one more electron to complete the K shell. Two hydrogen atoms share their electrons to form hydrogen molecule (H2), shown by two crosses between two hydrogen atoms. The one shared pair of electrons constitutes a single covalent bond between the two hydrogen atoms which is represented by a line between the two atoms. Other examples are Cl2, HCl, CH4, CCl4, H2O, NH3, CH3Cl, H2S, F2 etc.
(b) When two atoms, for example, oxygen atoms share two pairs of electrons, by contributing two electrons from each of the atoms, they make two covalent bonds. The double bond formed can be represented by two lines between two atoms. Example –Formation of oxygen (O2) molecule.
Why does carbon always form covalent bond?