1. USEFUL DEFINITIONS Element A substance which cannot be split into anything simpler by chemical means. Atom The smallest part of an element that can take part in a chemical reaction. Molecule The smallest particle of a compound (a combination of two or more elements). It is also the name given to the smallest part of those elements which do not exist as atoms in the free state i.e. hydrogen H 2 oxygen O 2 nitrogen N 2 fluorine F 2 chlorine C l 2 bromine Br 2 iodine I 2 N.B. ionic compounds (e.g. sodium chloride) do not exist as molecules.

2. USEFUL DEFINITIONS Ion The name given to any electrically charged atom or molecule. positively charged ions are known as cations negatively charged ions are known as anions Like charges repel but unlike (opposite) charges attract. If the sum of all the positive charges is equal and opposite to all the negative charges then the species will be neutral (no overall charge). Symbol A symbol represents one atom, or one mole, of an element.

3. Valency Is a numerical measure of the combining power of an atom / ion. Historically, it was the number of hydrogen atoms which will combine with one atom, or group of atoms. It is also the number of positive (+) or negative (-) charges on an ion. e.g. Mg +2, K +, S 2-

4. Valency may also be defined as the number of electrons that an atom donates or accepts to form the duplet state (i.e., 2 electrons in outermost shell) or octet state (i.e., 8 electrons in outermost shell). The valency of an element is always a whole number.

5. TABLE OF VALENCY valencyElement/ group losing elements 1All group 1 elements, hydrogen (H), Silver (Ag), ammonium (HN 4 ) 2All group 2 elements, Zinc (Zn), 3All group 3 elements 4Carbon C), Silicon ( Si),

6. ValencyElement/ group gaining electrons 1All group 7 elements, hydroxide (OH), Nitrate (NO 3 ), Hydrogen carbonate (HCO 3 ) 2 Oxygen (O), Sulphur (S), Sulphate (SO 4 ), Carbonate (CO 3 ) Valencies of some elements & Radicals

7. Elements With Variable Valencies Element SymbolValencyNomenclature Copper Cu1, 2 Copper [I] or Cuprous Copper [II] or Cupric Silver Ag1, 2 Silver [I] or Argentous Silver [II] or Argentic Mercury Hg1, 2 Mercury [I] or Mercurous Mercury [II] or Mercuric Iron Fe2, 3 Iron [II] or Ferrous Iron [III] or Ferric Tin Sn2, 4 Tin [II] or Stannous Tin [IV] or Stannic Lead Pb2, 4 Lead [II] or Plumbous Lead [IV] or Plumbic Phosphorus P3, 5 Sulphur S2, 4, 6

8. USEFUL DEFINITION Formula A formula represents one molecule of a compound, or the simplest ratio of the ions present. As with symbols, a formula represents a single particle or one mole of particles. The number of atoms or groups of atoms in a formula is given by putting a small number just below and behind the symbol(s). As the appearance of a symbol indicates one atom is present, a 1 isn’t written (you put NaBr not Na 1 Br 1 ). In some formulae brackets are used to avoid ambiguity. Aluminium sulphate has the formula Al 2 (SO 4 ) 3 to show that there are two Al’s to every three SO 4 ‘s. Without the brackets it would appear as though there were forty three O’s i.e. Al 2 SO 43.

9. Using valency to write chemical formulae The following steps are to be considered when writing chemical formulae Write each symbol Write the valency at the top right-hand side of each symbol Cross over the valencies Write the crossed-over valency at the bottom right of the other symbol. Where necessary simplify the ratio of valencies

10. Example what is the formula of aluminium oxide? Answer. 1. Al O 2. Al 3 O 2 3. Al 2 O 3 4. Al 2 O 3 Hence, the chemical formula of aluminium oxide is Al 2 O 3

11. Chemical formula of Calcium hydroxide 1. Ca OH 2. Ca 2 OH 1 3. Ca 1 OH 2 4. Ca(OH) 2 Hence, the chemical formula of calcium hydroxide is Ca(OH) 2,

12. chemical formula of Magnesium oxide 1. Mg O 2. Mg 2 O 2 3. Mg 2 O 2 4. Mg 2 O 2 5. MgO Hence, the formula of Magnesium oxide is MgO

13. Iron (III) Sulphate 1. Fe SO 4 2. Fe 3 SO 4 2 3. Fe 2 SO 4 3 4. Fe 2 (SO 4 ) 3 The chemical formula of Iron (III) Sulphate is Fe 2 (SO 4 ) 3

14. CONSTRUCTION OF FORMULAE It is useful to learn some formulae... Acidshydrochloric acid HClnitric acidHNO 3 sulphuric acid H 2 SO 4 ethanoic acidCH 3 COOH Gasesammonia NH 3 carbon dioxideCO 2 carbon monoxide COmethaneCH 4 sulphur dioxide SO 2

15. Write the formulae for the following compounds sodium sulphate zinc carbonate copper I nitrate Potassium chloride sulphuric acid Iron II nitrate Lead II carbonate Calcium hydroxide Magnesium sulphate

16. Practicalword equation 1. calcium carbonate + hydrochloric acid → calcium chloride + carbon dioxide + water 2. copper + silver nitrate = copper nitrate + silver 3. methane + oxygen = carbon dioxide + water 4. sulphuric acid + magnesium = magnesium sulphate + hydrogen 5. copper (II) carbonate = copper (II) oxide + carbon dioxide 6. iron (II) sulphate + sodium hydroxide = iron (II) hydroxide + sodium sulphate