HS Chemistry - Chemical Bonding

Polarity

Overview of The Page

This page will cover:

• What are dipoles?
• What is a molecule's polarity?

Molecules $including those of ionic and metal compounds$ have a property called polarity, which measures the difference in electronegativity found in the molecule. This is important for quite a few properties of molecules.

Dipoles

When molecules are formed, unless they consist entirely of the same atom $like how a fluorine molecule, F~2~, is made of 2 Fluorine atoms$, the molecule will be made up of atoms with different electronegativities. Electronegativity is the tendency of an atom to pull electrons towards its nucleus. In other words, atoms with higher electronegativities will pull electrons closer towards themselves.

In this example, Nitrogen $Element 7 on the Periodic Table$ has a lower electronegativity than Fluorine $Element 9 on the Periodic Table$. Therefore, Fluorine can pull its electrons closer towards its nucleus, making it smaller in size than Nitrogen. The main point here is that Fluorine is pulling its electrons closer towards its nucleus than Nitrogen is pulling its electrons towards its nucleus - Fluorine's nucleus is exerting greater force on its electrons, both its inner and its outer $valence$ electrons, than Nitrogen's nucleus.

If two atoms with different electronegativities bond together and create a molecule, then there is a difference in the electronegativities of the atoms. This difference between the atoms' electronegativities is called the polarity of the molecule.

In the example of Hydrogen Iodide $HI$, Hydrogen has an electronegativity of 2.1, while Iodine has an electronegativity of 2.5. That's an electronegativity difference of 0.4. Recall that in a covalent bond, both atoms involved in the bond will try to pull on the shared electrons, and will fight over them. Here, both Hydrogen and Iodine will fight over the shared electron, but since Iodine has a higher electronegativity, it will be able to pull the shared electrons closer towards itself. Thus, the Iodine atom will have a slightly higher concentration of electrons than the Hydrogen atom. This means that the Iodine atom will be slightly more negatively charged than the Hydrogen atom. This separation of charges between two bonded atoms is called a dipole.

To measure how large the dipole is, find the difference between the electronegativities of the bonded atoms. It is also useful to show which of the two atoms has the more positive charge, and which has the more negative charge. Additionally, larger arrows and symbols typically denote a greater dipole. An example has been given below:

This example shows the dipole of Hydrogen Iodide, with the bond between Hydrogen and Iodine shown by a dash. Below the atoms are symbols indicating which one is more negatively charged, and which is more positively charged. On top is an arrow pointing towards the more negatively charged atom, with a plus sign on top of the more positively charged atom. Note that when the dipole moment is so low $0.4$, it's usually not shown - this is just for the sake of an example.

Distinguishing Types of Bonds By Polarity

Non-polar covalent bonds: Occurs when the dipole between two bonded atoms is low $less than 0.5$. The dipole is so low that its effects aren't significant.

Polar Covalent Bonds: Occurs when the dipole between two bonded atoms is moderate $0.5 - 1.8$. The dipole is low enough that the electrons are shared between the atoms, but high enough that its effects are seen. The greater the difference in electronegativity is, the more polar the bond is.

Ionic Bonds: Occurs when the dipole between two bonded atoms is high $greater than 1.8$. The dipole is so high that the electrons just transfer over to the atom with higher electronegativity. Only occurs between a metal and a non-metal.

Metals can conduct electricity, and therefore their polarity won't be discussed here.

Molecule Polarity

If a diatomic molecule has a non-polar covalent bond, the molecule is said to be non-polar. If a diatomic molecule has a polar covalent bond, the molecule is said to be polar. Hydrogen Iodide, with an electronegativity difference of 0.4, would thus be a non-polar molecule.

$Since ionic compounds don't consist of separate molecules, their polarity is more complex, and therefore won't be discussed here$.

From this, it may seem that carbon tetrafluoride $CF~4~$ is a polar molecule, because the difference between carbon's electronegativity and fluorine's is 1.5. However, CF₄ is actually non-polar, because it is symmetrical.

Here's a diagram of CF₄:

All the dipoles not only have the same value, but they also point in opposite directions. Thus, they cancel each other out, and while the bonds between the carbon atom and each fluorine atom are polar covalent bonds, the molecule as a whole is non-polar.

The shape of the molecule isn't a factor in a diatomic molecule, because there's only one bond, but in a polyatomic molecule, with multiple bonds pointing in multiple directions, it becomes important. Are they pointing in opposite directions or are they just pointing in perpendicular directions? By how much? Usually, you'll only be asked to tell whether the molecule is polar or not. If the dipoles are in opposite directions, they'll cancel out, otherwise, they won't.

Molecule shape is covered on the next subpage, Electron & Molecular Geometry.