Intermolecular Forces

The forces that occur between the molecules.

What are Intermolecular Forces?

• Gases are widely spaced, randomly arranged particles.

• Liquids are close together, ordered particles, but with some empty spaces. They are more similar to solids than gases.

• Solids are closely packed, highly ordered particles.

What are Intermolecular Forces?

• Some compounds are liquid at room temp.• Some compounds are solid at room temp.• Some compounds are gaseous at room temp.

• This is because they have different intermolecular forces. There are different forces occurring between the molecules.

Intermolecular vs. Intramolecular

• Intermolecular forces are attractive forces that occur between molecules.

• Intramolecular forces are also known as bonds that occur between atoms in a molecule.

Intermolecular vs. Intramolecular

Dipole-Dipole• What is a dipole?– A polar compound that has one end that is

positively charged and the other end is negatively charged.

• Positive ends attract to negative ends.• When molecules with dipole

moments are put together, they orient themselves to take advantage of their positive & negative charges.

Dipole-Dipole

• The molecules with dipole moments can attract each other by lining up so that their positive & negative ends are close to each other.

• Dipole-dipole forces are very weak. They are only about 1% as strong as ionic or covalent bonds.

Hydrogen Bonding

• A hydrogen bond is a strong dipole-dipole force that occurs between molecules in which hydrogen is bound to a highly electronegative atom, such as oxygen.

• The most common hydrogen bond is found in water H2O.

Hydrogen Bonding

• The two factors that account for the strength of this force is– The polarity of the bond– The close proximity of the

dipoles because of the small size of the hydrogen atom.

Hydrogen Bonding• Water molecules tend to remain together in the

liquid state even at relatively high temperatures because a large amount of energy is required to overcome this force.

• This is because of the electronegativity value of oxygen.

• Thus water has a high boiling point.

London Dispersion Forces

• Even molecules without dipole moments must exert forces on each other.

• We know this because all substances – even the noble gases - exist in the liquid and solid states at very low temps.

• There must be forces to hold the atoms close together.

• The forces that exist among noble gas atoms and nonpolar molecules are called London Dispersion forces.

London Dispersion Forces

• We assume that the electrons of an atom are uniformly distributed about the nucleus,

• this is apparently not true at every instant.

London Dispersion Forces

• Atoms can develop a temporary dipolar arrangement of charge as the electrons move around the nucleus.

• This instantaneous dipole can then induce a similar dipole in a neighboring atom.

London Dispersion Forces

• The inter-atomic attraction is weak and short-lived.

• It can be very significant for large atoms and large molecules.

London Dispersion Forces

• The motion of the atoms must be slowed down before the weak London forces can lock the atoms into place to produce a solid.

• This is why noble gases have very low freezing points.

London Dispersion Forces

• Nonpolar molecules such as H2, N2, and I2 also attract each other by London Dispersion forces.

• These forces become more significant as the sizes of the atoms increase.

• Larger size means there are more electrons available to form the dipoles.

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