Intermolecular Forces

What are Intermolecular Forces (IMFs)?

Intermolecular forces or IMFs are the forces between separate molecules that either attract or repel them. These forces are created by positive and negative charges of the molecules. However, these forces are weak compared to intramolecular forces, which hold atoms within the molecules together.

IMFs can play a part in the physical properties of a substance. The stronger the IMFs the more strongly molecules stick together. Therefore chemicals with hstrength IMFs have lower boiling points than ones with higher IMFs.

Polar Molecules

Polar molecules do not have an overall charge; however, they have an uneven distribution of electrons. This creates parts of the molecule that have a slight positive charge and a slight negative charge. Opposite charges attract, so the positive side of one molecule will be attracted to the negative charge of the other.

Hydrogen Bonding

Hydrogen Bonding is a type of IMF that occurs when hydrogen is bonded to a very electronegative atom. These elements include: Fluorine, Nitrogen, Oxygen, or Chlorine. This creates a polar bond with the hydrogen developing a positive charge and the other element a negative charge.

This is the strongest type of IMF and holds the molecules together relatively strongly.

Hydrogen bonding prevalent in water and causes water molecules to stick together. It is responsible for its high boiling point, its liquid state at room temperature, and high surface tension.

Dipole-Dipole

A Dipole is another name for the separation of charge in a molecule. Dipoles cause molecules to act like a tiny magnets. There is a positive end (north pole) and a negative end (south pole). These are formed by polar bonds separating electrons. Not all polar bonds involve hydrogen. Other elements can form polar bonds, but these bonds are generally less polar than ones that include hydrogen. Therefore dipole-dipole bonds are not as strong as hydrogen bonds.

London Dispersion Forces (LDFs)

Molecules without any polar bonds can form temporary dipoles. This happens because of the random motion of electrons in their cloud around the molecule. By chance electrons can clump together creating a slight positive charge. LDFs are more prevalent in larger molecules with more atoms and hence more electrons. Additionally, the charge on another molecule can induce a temporary dipole on a molecule.

LDFs are the weakest type of IMF.

Practice!

IMF Worksheet.docx

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