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
• 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.

ATOMIC NUMBER AND ATOMIC MASS

• An Example from the Periodic Table

• Atomic Number - the number of protons in an unreacted atom
• Mass Number - the number of protons and neutrons together

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'.

• Electron rings close to the nucleus are filled first.

ELEMENTS AND COMPOUNDS

• Element - An element is a substance composed of the same type of atoms (e.g. gold Au, oxygen O₂).
• Compound - A compound is a substance made of more than one type of atom (e.g. water H₂O, carbon dioxide CO₂).
• Molecule - A molecule is the smallest particle of either an element or a compound.

INERT OR NOBLE GASES

• Inert or Noble Gases are unreactive gases. They do not corrode nor react.
• Examples of Noble Gases are:

• The electron rings of these unreactive gases are full, therefore they become stable.

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 Na¹⁺)
• 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.

COVALENT AND IONIC COMPOUNDS

• Covalent Compound - a compound where electrons are shared between the atoms (e.g. carbon dioxide CO₂)
• 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 (Na¹⁺).

COMMON ELEMENTS AND SYMBOLS TO LEARN

NAMING COMPOUNDS

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.

WORKING OUT FORMULAE OF IONIC COMPOUNDS
(THE CROSS-OVER METHOD)

• Step 1 - In the ionic compounds to be learnt in junior science, there are two parts to the ionic compound - the first is a positive ion (usually a metal e.g. Na¹⁺) 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. Na₁ Cl₁ or Mg₂ O₂), 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 CO₃).

EXAMPLES OF CHEMICAL NAMES OF COMPOUNDS

EXAMPLES OF NUMBERS AND TYPES OF ATOMS
IN VARIOUS ELEMENTS AND COMPOUNDS