Bio 2.2 Van der Waals Forces

to stay or not to stay that is the question

What does a Intramolecular Bond do?

hold atoms in a molecule together

What is a Intermolecular Bond?

• hold two or more molecules together

• much weaker and determine physical state of substance

What are the three types of intermolecular bonds and what is their name for them?

1. London forces

2. Dipole-dipole forces

3. Hydrogen bonds

Van der Waals forces

Tell me about London forces

• cause bonds that are formed due to a temporary unequal distribution of electrons in an atom

• Very weak, occur between small nonpolar molecules

• Each molecule is weakly attracted to its neighbour

• Cumulative effect of London forces become more significant in large nonpolar molecules

Why is methane a gas at room temperature?

because it is a London force

Why is octane a liquid at room temperature?

Since octane is larger, its has closer attractions meaning the molecules are closer rather than spread out like methane (gas)

Tell me about Dipole-dipole forces

• occur between polar molecules, like HCl

• Slightly positive end of one polar molecule is attracted to slightly

  negative end of another polar molecule

• Stronger than London forces

Tell me about Hydrogen Bonds

• dipole-dipole forces that form between electropositive H of one polar molecule and an electronegative N, O, or F of another

• Strongest intermolecular force

Note the differences between intramolecular and intermolecular bonds

Intramolecular	Intermolecular
Holds one molecule together	Holds many molecules together
Strong attraction between atoms	Weak attraction between molecules
Commonly called “bonds”	Known as Van der Waals forces
Determines if electrons are transferred or shared	Determines state of substance
Ionic, covalent, polar covalent	London forces, dipole-dipole, hydrogen bonds