Formula for Compounds (ending in –ide) Compounds who’s names end in –ide contain only 2 elements. Their chemical formula can easily be worked out by considering the valency of each element.

Chemical Formulae The valency (combining power) of an element, is the number of single covalent bonds it can form. The valency can be deduced from the group number: Group Number 1 2 3 4 5 6 7 Valency

Writing Formula - Steps Write the symbols Underneath, write the valency Swap them over and write numbers as a subscript. Cancel out any common factor Leave out ‘1’ if present.

Worked Example 2: Work out the formula for calcium chloride Ca Cl 2 1 Ca1 Cl2 CaCl2

Further Examples: Pg28 Q9 Boron fluoride Carbon sulphide Hydrogen fluoride Sodium sulphide Aluminium oxide 6. Magnesium phosphide 7. Hydrogen sulphide 8. Potassium oxide 9. Carbon chloride 10. Calcium hydride BF3 CS2 HF Na2S Al2O3 6. Mg3P2 7. H2S 8. K2O 9. CCl4 10. CaH2

Lesson Starter Write the formula for the following: Lithium sulphide Calcium nitride nitrogen oxide Magnesium hydride Sodium iodide phosphorous chloride Aluminium sulphide Carbon hydride Magnesium oxide Hydrogen oxide Li2S Ca3N2 N2O3 MgH2 NaI PCl3 Al2S3 CH4 MgO H2O

Ionic Substances In an ionic substance the valency is given by the charge on the ion. Transition metals can form a number of different ions, and so their valency is given by roman numerals. I = 1 II = 2 III = 3 IV = 4 V = 5 VI = 6 VII = 7

Worked example: Work out the formula for iron(III)oxide Fe O 3 2 Fe2 O3 Fe2O3

Further Examples: Iron(II) chloride Manganese(IV) oxide Lead(II) iodide Silver(II) fluoride Copper(I) oxide FeCl2 2. MnO2 3. PbI2 4. AgF2 5. Cu2O

Group Ions Ions containing 2 or more different types of atom are known as group ions. The formula for these can be found on page 8 of the data booklet. N.B. The charge is on the WHOLE GROUP of atoms The valency is equal to the number of charges. Always place the formula for the group ion in brackets.

Worked Example 1: Calcium Nitrate Symbol Valency Swap Ca(NO3)2

Example 2: Ammonium carbonate NH4 CO3 1 2 (NH4)2(CO3)1 (NH4)2CO3 Brackets can be removed ONLY if the subscript is 1

Further Examples: Magnesium hydroxide Ammonium phosphate Potassium chromate Aluminium sulphate Calcium hydrogensulphite Mg(OH)2 2. (NH4)3PO4 3. K2CrO4 4. Al2(SO4)3 5. Ca(HSO3)2

Mixed Questions: Yellow Book Page 29 Q 12 Int 2 Textbook Page 64 Q11 Group ion Practice: Yellow Book Page 29 Q 11 Int 2 Textbook Page 61 Q5&6 Mixed Questions: Yellow Book Page 29 Q 12 Int 2 Textbook Page 64 Q11

Formula from Prefixes In some cases the name of the compound tells us its chemical formula. This information is given in the form of a prefix. If only one prefix is given it can be assumed that only one atom of the other element is present. N.B. You do not need to ‘cross over’ if a prefix is given.

Prefix Meaning Mono- 1 Di- 2 Tri- 3 Tetra- 4 Penta- 5 Hexa- 6 Hepta- 7

Worked Examples Carbon monoxide: CO Carbon dioxide: CO2 Dinitrogen monoxide: N2O Phosphorus trichloride: PCl3

Further Examples Diphosphorus trioxide Silicon tetrachloride P2O3 SiCl4 V2O5 As4O6 UF6 Diphosphorus trioxide Silicon tetrachloride Divanadium pentoxide Tetraarsenic hexaoxide Uranium hexafluoride