Section 2 - Intermolecular Forces

Comparing Ionic and Covalent Compounds

Particles attract each other, so it takes energy to overcome the forcesholding them together. If it takes high energy to separate the particles ofa substance, then it takes high energy to cause that substance to go fromthe liquid to the gaseous state. The boiling point of a substance is a good measure of the strength of the forces that hold the particles together. Melting point also relates to attractive forces between particles. Most covalent compounds melt at lower temperatures than ionic compounds do.

** Nonmetals tend to bond to Nonmetals to form covalent bondsand Metals tend to bond to Nonmetals to form ionic bonds.

Oppositely Charged Ions Attract Each Other

Ionic substances generally have much higher forces of attraction thancovalent substances. Recall that ionic substances are made up of separateions. Each ion is attracted to all ions of opposite charge. For small ions,these attractions hold the ions tightly in a crystal lattice that can be disrupted only by heating the crystal to very high temperatures.

Intermolecular Forces Attract Molecules to Each Other

For covalent substances, forces that act between molecules are calledintermolecular forces. This forces is short-range and decrease rapidly as molecules get farther apart. Because the forces are effective only when molecules are near each other, they do not have much of an impact on gases. A substance with weak attractive forces will be a gas because there is not enough attractive force to hold molecules together as a liquid or a solid.

Dipole-Dipole Forces

In the positive end of one molecule attracts the negative end of a neighboring molecule. Bonds are polar because atoms of differing electronegativity are bonded together. The greater the difference in electronegativity in a diatomic molecule, the greater the polarity is.

H - F+ -

Draw the Dipoles for HF (higher electronegativity the receives the negative dipole.)

Add other HF molecules with proper dipole-dipole orientation

H - F+ - H - F+ -

The greater the difference in electronegativity, more polar the molecules are, the stronger the dipole-dipole forces between them, and thus, the higher the boiling point.

Which substancehas the greatestdipoles?

Hydrogen Bonds

Compare the boiling points of H2O and H2S, shown on the previous slide.

These molecules have similar sizes and shapes. However, the boiling point of H2O is much higher than that of H2S. A similar comparison of NH3

with PH3 can be made. The greater the polarity of a molecule, the higher the boiling point is. However, when hydrogen atoms are bonded to very electronegative atoms, the effect is even more noticeable.

Compare the boiling points and electronegativity differences of thehydrogen halides, shown below.

As the electronegativity difference increases, the boiling point increases. The boiling points increase somewhat from HCl to HBr to HI but increase a lot more for HF. This is due to do a special form of dipole-dipole forces, called a hydrogen bond.

Strong hydrogen bonds can form with a hydrogen atom that is covalentlybonded to very electronegative atoms of N, O, or F. Diatomic molecules of hydrogen with either N, O, F are said to have hydrogen bonds. Therefore havevery high attraction for nearby molecules and have high melting/boiling points.

Hydrogen bonds are between hydrogen or N,O, or F because nitrogen, oxygen,and fluorine all have a small radius and high electronegativity.

Hydrogen Bonding Explains Water’s Unique Properties

The way water molecules arrange themselves when solidifying is why waterbecomes less dense and expands when it freezes.

Ionic compounds have charges which hold it together when its a solid and polar covalent compounds have dipole charges to hold molecules together to create solids.

What charges hold together nonpolar molecules when they are a solid or liquid ?

London Dispersion Forces

You will only be required to know that these forces are extremely weak andincrease in strength as the size of the molecule increases. They are caused by wandering electrons which cause a temporary unequal balance of charge around the molecule.