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Writing formulae

Writing formulae. Objectives To learn how to write formulae for compounds Outcome: you should be able to write the formulae for inorganic covalent compounds and ionic compounds comprising single atom ions or polyatomic ions. Ionic compounds remember. Using valencies.

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Writing formulae

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  1. Writing formulae Objectives To learn how to write formulae for compounds Outcome: you should be able to write the formulae for inorganic covalent compounds and ionic compounds comprising single atom ions or polyatomic ions.

  2. Ioniccompoundsremember

  3. Using valencies We work out how many ions/atoms/molecules combine by looking at their valencies Both ions with valency 1. Combine 1:1 E.g. sodium and chlorine (sodium chloride) Formula = NaCl, An ion with a valency of 2 will combine with two of a valency of 1 E.g. Sodium and oxygen (sodium oxide) Formula = Na2O

  4. Using valencies continued Two ions with a valency of three will combine with three with a valency of two E.g. Aluminium plus oxygen = aluminium oxide Formula = Al2O3

  5. How do we know the valency? For single atomic ions it is easy Groups 1 – 3 = +1 - +3 Groups 5 – 7 = -3 - -1 Group 4 will be 4 and it depends on whether they give or receive as to whether it is positive or negative C = 4- Sn = 4+

  6. Transition metals These can vary e.g. copper 1+ or 2+; nickel 2+ or 3+. When describing compounds with transition metals you will usually be told which valency the metals is taking. If in doubt use 2+ because they all have 2 electrons in their s energy level The valency of metals in group 4 can also vary e.g. lead and tin can both be 2+ or 4+

  7. What will the formula be Magnesium sulphide Potassium iodide Aluminium chloride Sodium nitride Calcium bromide Copper (ii) oxide Iron (iii) oxide Tin (iv) oxide

  8. Polyatomic ions At the moment it is best just to learn them. There aren’t too many that you will come across. It is on the hand out. LEARN THEM for a quiz. Also learn how to name simple inorganic covalent compounds.

  9. Polyatomic ions in formulae If there is more than one in the compound you show this by putting them in brackets. E.g. Calcium Nitrate Ca(NO3)2 The subscript 3 shows all nitrate ions contain 3 oxygen atoms. The subscript 2 shows that because nitrate has a valency of 1 and calcium of 2: calcium will bind with 2 nitrate ions.

  10. Try these Copper (ii) sulphate Sodium carbonate Copper (i) nitrate Calcium carbonate Sodium sulphate Sodium bicarbonate Silver (ii) thiocynate Tin (iv) sulphate

  11. How to work out the valency of a polyatomic ion Sulphate SO4 Sulphur has a valency of -2 Oxygen has a valency of -2 Sulphur shares its 6 electrons with the four Oxygen atoms Two oxygen atoms get two extra electrons so have no charge. The other two get one each leaving an overall shortage of 1 on each or two in total. So sulphate will receive 2 more electrons giving it a charge of -2.

  12. If that’s too complicated Just learn the ones I’ve given you and memorise them.

  13. Practice: Name the compounds NaI CaBr2 BeO Na3 N Ca3P2 FeO CuCl2 PbBr4

  14. Practice (write the formulae) Lithium fluoride Calcium oxide Potassium sulphide Barium Iodide Iron (ii) oxide Iron (iii) sulphide Copper(ii) fluoride Tin (iv) bromide Aluminiumsulphide Magnesium phosphide Sodium nitrate Calcium sulphate Sodium carbonate Iron (ii) nitrite Potassium hydrogen carbonate Lead (iv) thiocyanate

  15. Covalentrecap Name Writeformulae

  16. Quiz on formulae I will quiz you on ? You must be able to: 1 name and write formulae for ionic compounds containing simple and poly atomic ions Name and write formulae for common covalent molecules.

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