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States of Matter and Intermolecular Forces

SECTION 10 - Liquids Solids and Intermolecular Forces


Most of us know the three states of matter.

But when discussing Intermolecular Forces, we're only concerned with the 2 condensed states of matter - liquids and solids.

The Three States of Matter

  • Gases - most compressible; the least dense
  • Liquids
  • Solids - least compressible; most dense

In this series of blog posts, we'll focus on the condensed states:

liquids and solids...

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Liquids and Solids

We know that the molecules of a liquid or solid are composed of atoms held together by "intramolecular forces" called bonds.

But individual molecules are aggregated together in a large sample (i.e. glass of water = billions upon billions of H2O molecules) by attractive forces called "intermolecular forces."

So... What are these "intermolecular forces," eh?

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What are Intermolecular Forces ?

Intermolecular Forces (IMF's)  =  forces that cause the aggregation of components of a substance to form a liquid or solid.

Intermolecular Forces are much weaker than bonds (intramolecular forces).

For example, boiling H2O causes individual molecules to:

Inter vs. Intramolecular Forces

As mentioned above, heat supplies enough energy to break the intermolecular forces holding the H2O(l) molecules together.

But the bonds themselves ( H—O—H ) are not broken.

To summarize,

➞ Intermolecular Forces occur between molecules.

➞ Intramolecular Forces (bonds) occur within molecules.

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Types of Intermolecular Forces

It's quite possible you've been told there are 3 types of intermolecular forces:

  • London Dispersion Forces (LDF, or Dispersion Forces)
  • Dipole-Dipole Forces (Dipole Forces)
  • Hydrogen Bonding (H-Bonds)

And this is "sort of" true.

Hydrogen Bonding is actually a special (stronger) subgroup within the Dipole-Dipole Forces group.

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Two Types of Intermolecular Forces

1. Dipole-Dipole Forces  =  only occur between polar molecules.

2. London Dispersion Forces  =  occur between all types of molecules, but these are the only forces holding nonpolar molecules and noble-gas atoms together.

London Dispersion Forces are also called:
  ➞ LDF, or
  ➞ Dispersion Forces, or
  ➞ van der Waals Forces.

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In my next blog post covering SECTION 10 - Liquids, Solids, and Intermolecular Forces,

We'll take a much closer look at these two Types of Intermolecular Forces.