1. The Chemical Earth (8.2.3)
Elements in Earth materials are present mostly as compounds because of interactions at the atomic level

2. Answers to 8.2.2 Element Outer shell electrons No. Shells Symbol
Atomic Number
Atomic Mass
Metal, semi-metal
non-metal
Sulfur 6 3 S 16
32.1 NM
Rubidium 1 5 Rb 37
85.5 M
Selenium 4 Se 34 79
Boron 2 B
10.8 SM
Arsenic As
74.9
Barium Ba 56
137.3
Krypton 8 Kr 36
83.8
Germanium Ge 32
72.6
Bromine 7 Br 35
79.9

3. Answers to 8.2.2 (contd.) Element Uses of element Chlorine
Sterilization of water.
Sodium
Heat transfer fluid in nuclear power plants.
Tungsten
Filaments in light globes, etc.
Gold
Jewellery, etc.
Mercury
Electrical switches, etc.
Zinc
Protecting iron (galvanizing).
Iron
Major building material.
Copper
Copper electrical wire.
Argon
Inert gas used in light bulbs and welding.
Helium
Carry weather balloons aloft.
Magnesium
Structural metal, flares, fireworks.

4. The Particle Theory
All matter is composed of particles. These particles are called atoms.
Solids – Particles in a solid are packed tightly together and locked firmly into position. They can vibrate in position but cannot move.
Liquids – Particles in liquids are packed tightly together, but they can slide over each other and shift position.
Gases – Particles of gases have little affinity for each other and move freely about.

5. Particles of a solid packed tightly together, cannot move.
Particles of a liquid, can move over each other.
Particles of a gas. Free to move about. Rapid movement.

6. Electron Shells

7. Electron Shells (contd.)
The lowest energy level is the “K” shell, it is the nearest to the nucleus. Electrostatic attraction at this level is greatest for the electrons.
As we move away from the nucleus into higher energy levels, nuclear attraction becomes less.
(See “Atomic Size” power point file with this unit)

8. Nobel Gases
Nobel gases have complete outer electron shells. Because of this, they are Inert or Un-reactive. Nobel gases are the only elements that can exist as single atoms and are written Ar, Ne, He, etc.. Gases such as nitrogen, oxygen, etc. must form molecules of two to remain stable. Groups of two atoms are called diatomic molecules. O2, Cl2, N2, etc. Are all diatomic molecules.

9. Bonding in Compounds
Ionic Bonds – Form when atoms lose or gain electrons. Ionic bonds form due to electrostatic attraction of the ions.
Covalent Bonds – Form when atoms share electrons.
Equal Sharing – Equal sharing forms covalent molecular* compounds.
Unequal Sharing – forms slightly polar compounds call polar covalent compounds.
*Molecules are electrically neutral arrangements of at least two atoms formed from strong covalent bonds.

10. Bonding in Compounds (contd.)
Lewis electron dot diagrams show only the outer shell of atoms and illustrate bonding.
The lone electron of the hydrogen atom is shown as an “x”. Oxygen has six electrons shown as a “dot”. The molecule of water is shown, with the oxygen sharing electrons with hydrogen and vice versa.
Oxygen is a highly electronegative atom and holds the electrons donated by hydrogen closely to itself. This produces a Polar Molecule, as shown to the left.

11. Lewis electron dot diagrams of ionic substances is slightly different.
Na * Cl  [ Na ]+ [Cl]-
* Mg * O  [Mg] 2+ [O] 2-

12. Questions
In terms of the particle theory, what is the difference between solids, liquids and gases.
In terms of size and bonding, what is the difference between argon, oxygen, and helium.
Draw Lewis electron dot diagrams for:
Carbon dioxide
Nitrogen
Magnesium chloride
Methane