Molecular Shape and Polarity

In a covalent bond like that of hydrofluoric acid, the electrons are pulled toward the more electronegative atom, indicated by a partial charge. Such bonds are called polar bonds.

The charge separation creates a vector called the bond dipole moment, which is indicated by the Greek letter µ. Its value is the product of the magnitude of the partial charges and the distance between them.

The vector points from the less to the more electronegative atom and indicates the bond dipole moment. Its length is proportional to the magnitude of the electronegativity difference between the two atoms. Most diatomic molecules containing atoms of different elements have dipole moments and therefore are polar molecules. 

Electrostatic potential maps indicate the high and low electron density regions in the compound with red and blue colors, respectively. Colors in between depict moderate electron density.

In polyatomic compounds, the net dipole moment is determined by the individual bond dipole moments and geometry of the compound.

Consider a water molecule with two polar bonds. It has a bent shape and is a polar molecule.

In contrast, a carbon dioxide molecule is linear. The two carbon-oxygen bonds are polar but are oriented in opposite directions, canceling out each other's dipole moment  and making the overall molecule nonpolar. 

Carbonyl sulfide molecules are also linear. However, the dipole moments of the carbon-oxygen and carbon-sulfur bonds do not cancel each other, and the molecule has a net dipole moment. 

Boron trifluoride is a trigonal planar compound. The dipole moments of the boron-fluorine bonds cancel each other owing to the molecular symmetry, and the compound is nonpolar. 

However, the dipole moments of the three polar bonds in trigonal pyramidal phosphorus trichloride molecules do not cancel each other, making it a polar compound.

Tetrafluoromethane is a tetrahedral molecule, which is nonpolar, as the dipole moments of the four identical polar bonds cancel each other. 

Fluoromethane is also a tetrahedral molecule. However, it possesses a net dipole moment as the C-F bond has a large dipole moment compared to the C-H bonds, and the bond dipole moments do not cancel each other.

In an electric field, polar molecules align the positive end toward the negative plate and the negative end toward the positive plate. In contrast, nonpolar molecules stay unaffected by an electric field. 

Generally, polar solvents dissolve polar solutes, and nonpolar solvents dissolve nonpolar solutes because similar types of molecules tend to have more favorable interactions. Water is polar and easily dissolves polar compounds such as sucrose, commonly known as table sugar. Oil is nonpolar and remains immiscible in water.