1. Gases – Structure and Properties

2. A Model for Gases
Particles in gases move in straight lines at very high speeds
They change direction only when they collide with each other or the walls of the container
When the particles collide with the walls of the container pressure is produced

3. Kinetic energy of gases
They move at speeds proportional to the kinetic energy they possess
Kinetic energy is the energy possessed by moving objects
KE = ½mv2
The larger the mass, the more energy it has
The faster a mass is travelling, the more energy it has

4. Postulates
Gases are comprised of tiny particles whose size is negligible compared to the distance separating them.
Particles in gases are in constant, random motion. They often collide with one another and the walls of the container. It is the collisions with the walls that create gas pressure.
Molecular collisions are elastic. While particles may gain or lose energy, there is no net loss or gain of energy from the system, the energy is transferred.
At a given temperature, the average kinetic energy of the molecules is constant.
As temperature increases, so do the average velocity and kinetic energy of the molecules.

5. Intermolecular forces
Relatively weak forces of attraction between individual molecules, often called Van Der Waals’ Forces.
Dispersion forces are the weakest type and exist between all molecules.
The boiling points of the Noble Gases increase with the atomic mass. This means that the attractive forces are stronger between larger atoms (gravity example).
Electrons in the electron cloud repel each other and try to stay as far apart as possible. However, due to their constant motion, they may not always be positioned symmetrically around the nucleus. When this happens dipoles form.
This dipole can induce a dipole in a neighbouring atom.
The attraction between these two dipoles is called a dispersion force.

7. The Shape of Molecules
All molecules have a definite 3D shape. It is determined by the number of electron groups on the central atom and on the number of atoms bound to it.
The number of electron groups is equal to the number of atoms bonded to the central atom plus the number of non-bonded electron pairs.
VSEPR theory suggests that electron groups will try and be as far apart as possible.
Eg CO2 the central atom only has two electron groups O=C=O with bond angles of 180° between groups. This is a linear structure.
Eg BCl3 the central atom as three electron groups Cl-B-Cl with bond angles of 120° between groups. This is trigonal planer structure.
Eg CH4 the central atom as four electron groups H-C-H with bond angles of ° between groups. This is tetrahedral structure. Cl H H

8. The exceptions…
Where there are un-bonded electron pairs, these electrons will repel each other more strongly and thus have slightly greater bond angles. This forces the bonded electrons to have slightly smaller bond angles.
See page 108 in Skinny Chem Text…