Section 2 - Intermolecular Forces Comparing Ionic and Covalent Compounds Particles attract each other, so it takes energy to overcome the forces holding them together. If it takes high energy to separate the particles of a substance, then it takes high energy to cause that substance to go from the 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 bonds and 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 than covalent substances. Recall that ionic substances are made up of separate ions. 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 called intermolecular 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 ++ -- ++ --

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 substance has the greatest dipoles?

Hydrogen Bonds Compare the boiling points of H 2 O and H 2 S, shown on the previous slide. These molecules have similar sizes and shapes. However, the boiling point of H 2 O is much higher than that of H 2 S. A similar comparison of NH 3 with PH 3 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 the hydrogen 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 covalently bonded 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 have very 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 water becomes 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 and increase 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.