4. Electron Sharing and      Covalent Bonds  
     Carbon Compounds

This very simple but useful way of predicting molecular shapes has been given the grandiose name of the valence-shell electron-pair repulsion theory, or VSEPR theory, but essentially it is nothing more than common sense. We shall develop simple VSEPR theory as it is needed, and will find that this theory accounts for almost all of the observed geometries of molecules.

The electronegativities of carbon and hydrogen are almost the same, 2.5 and 2.1, respectively. Electrons in the C-H bonds are shared almost equally by the two atoms, with little tendency to shift toward either C or H. The bonds are said to be nonpolar, because there is no accumulation of positive charge at one end and negative charge at the other caused by the movement of the bonding electron pair toward one atom. In contrast, the bond in H-F is quite polar because the high electronegativity of F pulls the bonding electron pair toward F, leaving the molecule with a positive charge on the H atom and a negative charge on the F atom.

Most of the forces between molecules are electrostatic, caused by attractions between the positive and negative charges on different parts of the molecule.

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