IntermolecularForces

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Intermolecular Forces

Have you ever thought why atoms or molecules hangout together in the condensed phase (i.e. liquid or solid phase) and not in the gas phase. What is keeping the molecules together???

For example, at room temperature, why is sodium chloride (table salt, Na Cl) a solid, water (H₂O) is a liquid and oxygen (O₂) is gas?

It is due to the bonding in the compound/element(s)including the distribution of electrons. It is called INTERMOLECULAR FORCES (IMF).

As the word implies, IMF are the forces between molecules. The greater the forces between molecules, the greater the energy needed for the two molecules not to relate with the other molecule (i.e. free from the affect of the other molecule, or in the gas phase).

The order of Intermolecule forces are as following (from lowest IMF to highest IMF),

London Dispersion Forces (LDF) - for two non-polar molecules.
Dipole-Dipole - for two polar molecules except Hydrogen bonding
Hydrogen bonding - for polar molecules that have the following bonds, O-H, N-H, F-H
Don't use until Solution Chemistry: Ion-dipole - an ion and a dipole molecule (e.g. water and sodium ion,Na¹⁺)
Ionic bonds - for compounds with ionic bonds (e.g. ionic compound)
Metallic bond (same as ionic bond) - for metals bonding together or with themselves

For London Dipersion Forces, the electrons in molecules are on, average,evenly distributed throughout the molecule. But in any given billionth of a second, the electrons can be unevenly distributed in the molecule. Therefore, for that billionth of a second, there are partially negative and positive areas in the molecules that will attract (forces) to another molecule and the two molecules could condense.

So what factors affect the LDF, the greater the molar mass (i.e. number of electrons), the greater the LFS since with more electrons there is a higher probability that the electrons are unevenly distributed.