How Do We Predict If a Molecule Is Polar or Nonpolar

In Section 1.2C, we used the terms polar and dipole to describe a covalent bond in which one atom bears a partial positive charge and the other bears a partial negative charge. We also saw that we can use the difference in electronegativity between bonded atoms to determine the polarity of a covalent bond and the direction of its polarity. We can now combine our understanding of bond polarity and molecular geometry (Section 1.3) to predict the polarity of molecules. 

A molecule will be polar if (1) it has polar bonds and (2) the vector sum of its bond dipoles is zero (i.e., the bond dipoles cancel each other). Consider first carbon dioxide, COg, a molecule with two polar carbon-oxygen double bonds. Because carbon dioxide is a linear molecule, the vector sum of its two bond dipoles is zero therefore, this molecule 

Ammonia has three polar N — H bonds, and because of its geometry, the vector sum of their bond dipoles does not equal zero. Thus, ammonia is a polar molecule. 

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