Intermolecular Attractions

Types of Intermolecular Attractions

There are three types of intermolecular attractions:

• London Forces

• Dipole-dipole

• Hydrogen bonds

Polarity of molecules

To understand the different types of intermolecular attractions we need to recognize that molecules may be either polar or non-polar.

POLAR MOLECULES have a positive side and a negative side due to the uneven distribution of electrons in the molecule. Polar molecules are also known as DIPOLES.

NON-POLAR MOLECULES do NOT have a positive and negative side because the electrons are evenly distributed among the atoms making up the molecule.

Polar molecules

Polar molecules are molecules that have a positive end and a negative end. These molecules consist of atoms that are bonded together by polar covalent bonds.

The bonding electrons are pulled closer to the atoms in the molecule with the greatest electronegativity. This makes one end of the molecule slightly negative, while the other end of the molecule is slightly negative.

Non-polar molecules

In a non-polar molecule, the electrons are distributed evenly throughout the molecule.

If a molecule is made up only of non-polar bonds, then the molecule will be non-polar.

Non-polar molecules (2)

However, some non-polar molecules may be made up of polar bonds, if the molecule is symmetrical. The symmetrical shape insures an even distribution of the electrons even when the bonds making up the molecule are polar covalent (have unequal sharing).

For example, in carbon dioxide the oxygen and carbon form polar covalent bonds, however, due to the symmetry of the molecule the separations of charge cancel out making the molecule non-polar. A NON-POLAR MOLECULE DOES NOT HAVE A + OR – END.

Centers of Charge

• In a non-polar molecule the center of positive and negative charge are at the same point in the molecule.

Centers of Charge 2

• While in a polar molecule, the centers of positive and negative charges do NOT cancel out leaving a positive and negative end to the molecule.

Center of Charge 3

• This is analogous to the difference between the geographic center of the US and the population center of the US as seen in this diagram:

Do you think the US will ever be non-polar in this regard?

London Forces

The London forces are the weakest inter-molecular attractions. They are the forces that are responsible for holding non-polar molecules together.

Molecules held together by the London forces include all 7 diatomic elements, CO2, and CH4 (methane).

Substances whose molecules are held together by the London forces tend to have very low boiling points (are gases at room temperature) because the London forces aren’t strong enough to hold the molecules together.

Dipole-Dipole Interactions

The Dipole-dipole forces are stronger than the London forces. They are the forces that are responsible for holding polar molecules together.

Molecules held together by the dipole-dipole attractions include all most polar molecules such as HCl and CO.

Substances whose molecules are held together by dipole-dipole attractions tend to have higher boiling points than molecules held together by the weaker London forces.

Hydrogen bonds

The Hydrogen bonds are the strongest intermolecular attractions. They are the forces that are responsible for holding molecules that have hydrogen bonded to very electronegative elements such as nitrogen, oxygen, and fluorine. Hydrogen bonds are an unusually strong form of dipole-dipole attractions.

Molecules held together by Hydrogen bonds include water (H2O), hydrogen fluoride (HF) and ammonia (NH3).

Substances whose molecules are held together by hydrogen bonds tend to have higher boiling points than molecules held together by the other intermolecular forces..

All non-polar molecules (symmetrical) are held together by London Forces. Boiling pts. Increase as molecules become larger

Group 16 – hydrogen compounds

Polar molecules – held together by dipole-dipole attractions. Boiling pts again increase as molecules become larger. Except water!

The unusually high boiling pt of water is caused by the hydrogen bonds that hold the water molecules together.

Crystalline solids fall into one of four categories:

Type of Solid

Forces holding particles together

Properties Examples

Ionic IonicHigh Melting Point, Brittle, Hard, Nonconductors as solid, good conductors as liquids or when dissolved in water.

NaCl, MgO

Molecular

Hydrogen Bonding, Dipole-Dipole, London Dispersion

Low Melting Point, Nonconductors H2, CO2

Metallic Metallic Bonding

Variable Hardness and Melting Point (depending upon strength of metallic bonding), Good conductors as solids and liquids

Fe, Mg

Covalent Network

Covalent Bonding

High Melting Point, Hard, Nonconductors

C (diamond), SiO2

(quartz)

Network solids

Ice – a molecular solid

When water molecules go from the liquid state to the solid state they line up to bring the H and O atoms in nearby molecules as close together as possible. This leaves spaces between the molecules making ice LESS dense than liquid water.