3. AN ATOM Atom - smallest unit of all matter, that is composed of 3 sub-atomic particles called protons, electrons and neutrons Proton - the 'heavy' positively-charged particle in the nucleus of an atom Electron - the very 'light' negatively-charged particle that orbits the nucleus of an atom Neutron - the 'heavy' uncharged particle in the nucleus of an atom

4. AN ATOM Uncharged or unreacted atoms have the same number of positive protons and negative electrons. Approximate size of atoms - Millions of atoms could fit on the sharp point of a needle. Also, if you imagine that an atom is the size of an oval, a proton and a neutron would be the size of footballs in the middle of the oval, and the electron would be the size of a rice grain racing around the running lane.

5. ATOMIC NUMBER AND ATOMIC MASS Atomic Number - the number of protons in an unreacted atom Mass Number - the number of protons and neutrons together

6. ATOMIC DIAGRAMS Protons and neutrons are in the nucleus. Protons are p+ Neutrons are n Electrons orbit the nucleus in electron levels or 'rings'. Electrons are e- A limited number of electrons are situated in each electron 'ring'. First ring - maximum of 2 electrons Second ring - maximum of 8 electrons Third ring - maximum of 8 electrons Fourth ring - maximum of 18 electrons Electron rings close to the nucleus are filled first.

7. ELEMENTS AND COMPOUNDS Element - An element is a substance composed of the same type of atoms (e.g. gold Au, oxygen O 2 ). Compound - A compound is a substance made of more than one type of atom (e.g. water H 2 O, carbon dioxide CO 2 ). Molecule - A molecule is the smallest particle of either an element or a compound.

8. INERT OR NOBLE GASES Inert or Noble Gases are unreactive gases. They do not corrode nor react. Examples of Noble Gases are: He - Helium Ne - Neon Ar - Argon Kr - Krypton Xe - Xenon Rn - Radon The electron rings of these unreactive gases are full, therefore they become stable.

9. IONS (CHARGED ATOMS) When atoms react, they may either gain or lose electrons. Electrons have a negative charge. An atom gaining or losing electrons will get an overall charge. Positive Ions are atoms that have lost electrons (e.g. sodium Na 1+ ) Negative Ions are atoms that have gained electrons (e.g. chlorine Cl 1- ) In chemical reactions, atoms tend to gain or lose electrons to resemble the electron numbers of the stable Noble Gases.

10. COVALENT AND IONIC COMPOUNDS Covalent Compound - a compound where electrons are shared between the atoms (e.g. carbon dioxide CO 2 ) Ionic Compound - a compound formed from the attraction between positive and negative ions. For example in the ionic compound sodium chloride NaCl, the chlorine ion (Cl 1- ) gains one electron that was given by the sodium ion (Na 1+ ).

11. VALENCY TABLE Valency - the charge of an ion or radical which has either lost or gained electrons Note that metals lose electrons easily to become positive ions. This is why most metals are good conductors of electricity.

12. WORKING OUT FORMULAE OF IONIC COMPOUNDS (THE CROSS-OVER METHOD) Step 1 - In the ionic compounds, there are two parts to the ionic compound - the first is a positive ion (usually a metal e.g. Na 1+ ) and the second is a negative ion (e.g. Cl 1- ). Step 2 - Using the valency table, write the two ions and their valencies. Step 3 - Now ignore the positive and negative signs. Cross-over the top valency number to the bottom of the other ion symbol. Do this for both. Step 4 - Write the completed formulae with those same numbers at the bottom. Step 5 - If the numbers on each part are the same (e.g. Na 1 Cl 1 or Mg 2 O 2 ), ignore them and rewrite the formulae without them (e.g. Na Cl or Mg O). Step 6 - Brackets may be used around radicals (groups of atoms that are charged e.g CO 3 ).