When you start learning about chemistry in Year 12, you'll come across something important: the differences in how polar and non-polar molecules interact with each other. These differences help us understand things like boiling points, melting points, and solubility.

Polar Molecules

1. What Are They?: Polar molecules have a big difference in the way their atoms attract electrons. This makes the distribution of electrons uneven. Because of this, one end of the molecule becomes slightly negative (δ-) and the other end becomes slightly positive (δ+). This is called a dipole moment.

2. How They Interact:

   • Dipole-Dipole Interactions: This is the main way polar molecules interact. The positive end of one polar molecule pulls in the negative end of another.
   • Hydrogen Bonds: This is a special and stronger type of dipole-dipole interaction. It happens when hydrogen bonds with really electronegative atoms like oxygen, nitrogen, or fluorine. A good example is water ($H_2O$). Its hydrogen bonds make it boil at a higher temperature compared to non-polar molecules of similar size.

Non-Polar Molecules

1. What Are They?: Non-polar molecules have an even spread of electrons. This happens because they have atoms that attract electrons similarly, or their shape is symmetrical enough to cancel out any dipole moments.

2. How They Interact:

   • London Dispersion Forces: This is the main force that non-polar molecules experience. These are weak interactions that occur because of temporary changes in electron density. For example, in molecules like methane ($CH_4$), these are the only interactions present.

3. Comparing Strengths: Although London dispersion forces can be stronger in larger non-polar molecules, they are still generally weaker than the forces in polar molecules, like dipole-dipole interactions and hydrogen bonds. That’s why polar substances often have higher boiling points than non-polar substances that are similar in size.

Summary

• Strength Order: The strength of these molecular interactions usually follows this order:
  • Hydrogen bonds (strongest)
  • Dipole-dipole interactions
  • London dispersion forces (weakest)

Because of these differences, polar and non-polar molecules act differently when they meet. For example, oil is non-polar and water is polar, and they don’t mix because the strong hydrogen bonds in water overpower the weaker forces in oil.

Knowing how these interactions work helps us predict how different chemicals will behave. Whether it's figuring out how substances react, their states at different temperatures, or why some things dissolve and others don’t, understanding the strength of these interactions is really important in chemistry.