Chemistry for Liberal Studies - Forensic Academy / Dr. Stephanie R. Dillon

Structure Based Molecular Properties

Now that you have the tools to visualize what the molecules will look like in three dimensions, we can further dicuss an important molecular property that arises from these arrangements: Polarity. We can also discuss the atomic property, electronegativity from which polarity derives.

Electronegativity

Electronegativity is actually an atomic property. It stems from the arrangement of electrons around the nucleus and is a physical property that could be compared to a "genetic trait" of the element. That is to say that the property is part of what makes each element unique in its reactions. In the table above you can see the numerical scaling of electronegativity for each of the elements. What is easily observable is that the electronegativity values increase from left to right and from bottom to top in the periodic table stopping just shy of the noble gases. Fluorine is the most electronegative atom with a value of 4 and Francium is the least electronegative with a value of 0.7.

So how does this property affect the bonding of molecules? Well, we can now relate the type of bond that forms to the electronegativity values of the atoms involved.

There are three types of bonding that we can define in terms of electronegativity. Two of those types we have seen before: Covalent and Ionic. The third is called Polar Covalent and we will explain it further in a short while along with our explanation of Polarity.

So starting with the Nonpolar Covalent bonds we are already familiar with, we see that this bond occurs when there is equal sharing (between the two atoms) of the electrons in the bond. Diatomic molecules like H2 or F2 are good examples. In terms of electronegativity, a maximum difference of 0.2 - 0.5 in the values of the atoms results in a nonpolar covalent structure. The most common nonpolar covalent bonds are those between carbon and hydrogen: C has an electronegativity of 2.5 and H is 2.1 for a difference of 0.4.

Carbon Hydrogen bond

The other type of bond we are already familiar with is the ionic bond. This type of bond occurs when there is complete transfer (between the two atoms) of the electrons in the bond. Substances such as NaCl and MgCl2 are the usual examples. The rule is that when the electronegativity difference is greater than 2.0, the bond is considered ionic.

So there is a broad range of values (0.6 to 1.9) in the electronegativity scale left to consider and this is where we introduce the third type of bonding: Polar Covalent. This type of bond occurs when there is unequal sharing (between the two atoms) of the electrons in the bond. Water is the most common example of polar covalent bonding:

Notice the hot pink and green sections. These are there to show that while the structure is covalent and the hydrogen and oxygen atoms are sharing electrons, they are not sharing them equally. The oxygen (EN = 3.5) is far more electronegative than the hydrogen (EN= 2.1) and therefore the electrons are drawn to and concentrate around the oxygen, leaving the nucleus of the hydrogen atoms somewhat bare. This uneven distribution of electrons leads to regions of partial positive (δ+) and partial negative (δ-) charge around the molecule creating what is called a Dipole.

Dipoles are indicated by arrows as shown with the arrow pointing the direction the electrons are flowing. The existence of these dipoles along the bonds in a molecule leads (in most cases) to the overall molecule being defined as Polar.

Polarity

When a covalent compound contains polar bonds it has a high likelihood of being polar. There is one thing that can negate the polarity and that is symmetry. Now you see why it was important for you to be able to see the molecules in 3D before introducing this concept. Letís use an example to make the point about structure negating polarity.

Carbon Tetrafluoride is a nonpolar covalent compound. If we look at the bonds individually, Carbon has an electronegativity of 2.5 and fluorine has an electronegativity of 4.0. The difference of 1.5 says that each C-F bond is very polar, but when put into the structure the symmetry cancels this polarity out and the overall structure is nonpolar.

Remember that this is a tetrahedral structure in which all of the bonds are equidistant from each other. You could think of it as a tug of war between quadruplets. No one wins since they are all of equal strength in pull.

So what does being polar do for a molecule? In solution, it determines whether or not a molecule will be soluble and in what kinds of solvents. Polar molecules like to be with other polar molecules so they would dissolve in water but not in a very non-polar organic solvent like hexane:

Hexane (C6H14)

We can use this information many times to separate a substance of interest from other nonpolar substances in a mixture. Much the same way you separate dirt from your clothes as you wash them. Nonpolar detergent molecules surround the nonpolar dirt particles and separate them from your clothes.

Soap molecules are long nonpolar chains of carbon and hydrogen with a polar cap of sulfate or the like.

In water the nonpolar portion of the soap molecules attach themselves to other nonpolar molecules like dirt and form micelles. Micelles are structures in which the nonpolar (hydrophobic = water hating) side of the molecule are inside and the polar (hydrophilic = water loving) part of the molecule is on the outside. These micelles form around the dirt particles and remove them from the surface you are trying to clean. The polar heads of the soap molecules allows the dirt to be supported by the water around it and removed (washed away).

Polar and ionic compounds are soluble in water while nonpolar covalent molecules are not. There is a special category of polar covalent structures that needs to be further discussed: Acids and Bases.