Chemical Formulas

1. Chemical Formulas

2. Water is H2O.

3. Water is H2O.How do we decide on these strange-looking symbols for chemicals?

4. First of all we need to know the symbols for the elements which make up our universe.There are about ninety of these:-

5. I’ve chosen twenty of them:- H hydrogenC carbonN nitrogenO oxygenNa sodiumMg magnesiumAl aluminiumSi siliconP phosphorusS sulphurCl chlorineK potassium Ca calciumCr chromiumFe ironCu copperZn zincBr bromineAg silverI iodine Mostly the symbol is the first one or two letters of the name

6. Many ordinary compounds are made from atoms of just two or three of these elements combined together. The formula shows the ratio of the numbers of these atoms.

7. Many ordinary compounds are made from atoms of just two or three of these elements combined together. The formula shows the ratio of the numbers of these atoms.Water, H2O, has 2 hydrogens to 1 oxygen. These are linked to make one molecule of water.

8. Many ordinary compounds are made from atoms of just two or three of these elements combined together. The formula shows the ratio of the numbers of these atoms.Water, H2O, has 2 hydrogens to 1 oxygen. These are linked to make one molecule of water.Carbon dioxide, CO2, has 2 oxygens for each carbon.

9. These two substances consist of small particles called molecules, where just 3 atoms are linked together.The CO2 molecule is linear CO2 O=C=Owhilst the water molecule is V-shaped or bent H2O H H O

10. Not all compounds are made of molecules, some have electrically charged groups of atoms called ions.

11. Not all compounds are made of molecules, some have electrically charged groups of atoms called ions.One of these is aluminium sulphate, known as alum, whose formula is Al2(SO4)3 and which contains positive aluminium ions and negative sulphate ions, the SO4 part of the formula.

12. There are a small number of these poly-atomic ions whose formula we will need to know. Their names may be familiar:-sulphate SO42-nitrate NO3-carbonate CO32-hydroxide OH - where the negative signs indicate the electrical charge

13. The question now is:- “How do we know how many of each atom are needed?”.

14. The question now is:-“How do we know how many of each atom are needed?”. It turns out that atoms of each element (or poly-atomic ion) have a number associated with them which determines how they all link up together. This is sometimes called the valency or valence.

15. The question now is:-“How do we know how many of each atom are needed?”. It turns out that atoms of each element (or poly-atomic ion) have a number associated with them which determines how they all link up together. This is sometimes called the valency or valence.Here are the numbers for the twenty four atoms and ions we have met so far.

16. H hydrogen +1C carbon ±4N nitrogen -3O oxygen -2Na sodium +1Mg magnesium +2Al aluminium +3Si silicon ±4 P phosphorus -3S sulphur -2Cl chlorine -1K potassium +1Ca calcium +2Cr chromium +3Fe iron +2 or +3Cu copper +2Zn zinc +2Br bromine -1Ag silver +1I iodine -1 What a difficult set of numbers to learn! Or are they?

17. Let’s re-arrange the list H hydrogen +1 Na sodium +1 K potassium +1 Mg magnesium +2 Ca calcium +2 Al aluminium +3 Cr chromium +3 Fe iron +2 or +3 Cu copper +2 Zn zinc +2 Ag silver +1 Cl chlorine -1 Br bromine -1 I iodine -1 O oxygen -2 S sulphur -2 P phosphorus -3 N nitrogen -3 C carbon ±4 Si silicon ±4 NO3 nitrate -1 OH hydroxide -1 CO3 carbonate -2 SO4 sulphate -2

19. So how do we use this table?

20. So how do we use this table?Here are a few simple steps to follow in going from a compound’s name to its formula:-

21. So how do we use this table?Here are a few simple steps to follow in going from a compound’s name to its formula:-I’ll use calcium chloride as a example

22. So how do we use this table?Here are a few simple steps to follow in going from a compound’s name to its formula:-I’ll use calcium chloride as a example1 write the symbols implied by the name

23. So how do we use this table?Here are a few simple steps to follow in going from a compound’s name to its formula:-I’ll use calcium chloride as a example1 write the symbols implied by the nameCa and Cl

24. So how do we use this table?Here are a few simple steps to follow in going from a compound’s name to its formula:-I’ll use calcium chloride as a example1 write the symbols implied by the nameCa and Cl (compounds of two elements usually have a name ending in –ide, for three elements it’s usually -ate)

25. So how do we use this table?Here are a few simple steps to follow in going from a compound’s name to its formula:-I’ll use calcium chloride as a example1 write the symbols implied by the nameCa and Cl(compounds of two elements usually have a name ending in –ide, for three elements it’s usually -ate)2 Find the numbers on the list

26. Let’s re-arrange the list H hydrogen +1 Na sodium +1 K potassium +1 Mg magnesium +2 Ca calcium +2 Al aluminium +3 Cr chromium +3 Fe iron +2 or +3 Cu copper +2 Zn zinc +2 Ag silver +1 Cl chlorine -1 Br bromine -1 I iodine -1 O oxygen -2 S sulphur -2 P phosphorus -3 N nitrogen -3 C carbon ±4 Si silicon ±4 NO3 nitrate -1 OH hydroxide -1 CO3 carbonate -2 SO4 sulphate -2

27. So how do we use this table?Here are a few simple steps to follow in going from a compound’s name to its formula:-I’ll use calcium chloride as a example1 write the symbols implied by the nameCa and Cl(compounds of two elements usually have a name ending in –ide, for three elements it’s usually -ate)2 Find the numbers on the listCa+2 and Cl-1

28. So how do we use this table?Here are a few simple steps to follow in going from a compound’s name to its formula:-I’ll use calcium chloride as a example1 write the symbols implied by the name Ca and Cl (compounds of two elements usually have a name ending in –ide, for three elements it’s usually -ate)2 Find the numbers on the list Ca +2 and Cl -13 The rule now is that “The numbers in the compound must add to zero”.

29. So how do we use this table?Here are a few simple steps to follow in going from a compound’s name to its formula:-I’ll use calcium chloride as a example1 write the symbols implied by the nameCa and Cl (compounds of two elements usually have a name ending in –ide, for three elements it’s usually -ate)2 Find the numbers on the listCa+2 and Cl-13 The rule now is that “The numbers in the compound must add to zero”. one Ca (+2) and two Cl (-1) will add to 0.

30. So how do we use this table?Here are a few simple steps to follow in going from a compound’s name to its formula:-I’ll use calcium chloride as a example1 write the symbols implied by the nameCa and Cl(compounds of two elements usually have a name ending in –ide, for three elements it’s usually -ate)2 Find the numbers on the listCa+2 and Cl-13 The rule now is that “The numbers in the compound must add to zero”. one Ca (+2) and two Cl (-1) will add to 0.4 And so the formula is CaCl2

31. 2 Now let’s try aluminium iodide1 Al and I

32. 2 Now let’s try aluminium iodide1 Al and I2 Al +3 and I -1

33. 2 Now let’s try aluminium iodide1 Al and I2 Al +3 and I -13 one Al will need 3 I

34. 2 Now let’s try aluminium iodide1 Al and I2 Al +3 and I -13 one Al will need 3 I4 formula is AlI3

35. 2 Now let’s try aluminium iodide1 Al and I2 Al +3 and I -13 one Al will need 3 I4 formula is AlI33 sodium oxide

36. 2 Now let’s try aluminium iodide1 Al and I2 Al +3 and I -13 one Al will need 3 I4 formula is AlI33 sodium oxide1 Na and O

37. 2 Now let’s try aluminium iodide1 Al and I2 Al +3 and I -13 one Al will need 3 I4 formula is AlI33 sodium oxide1 Na and O2 Na +1 and O -2

38. 2 Now let’s try aluminium iodide1 Al and I2 Al +3 and I -13 one Al will need 3 I4 formula is AlI33 sodium oxide1 Na and O2 Na +1 and O -23 this time we need two Na for one O

39. 2 Now let’s try aluminium iodide 1 Al and I 2 Al +3 and I -1 3 one Al will need 3 I 4 formula is AlI33 sodium oxide 1 Na and O 2 Na +1 and O -2 3 this time we need two Na for one O 4 formula is Na2O

40. 2 Now let’s try aluminium iodide1 Al and I2 Al +3 and I -13 one Al will need 3 I4 formula is AlI33 sodium oxide1 Na and O2 Na +1 and O -23 this time we need two Na for one O4 formula is Na2Onotice that in the name and the formula, the metal or hydrogen (things which have the positive numbers) come first.

41. A few more examples3 calcium sulphate 1 Ca and SO4

42. A few more examples3 calcium sulphate 1 Ca and SO42 Ca +2 and SO4 -2

43. A few more examples3 calcium sulphate 1 Ca and SO42 Ca +2 and SO4 -23 one Ca for one SO4

44. A few more examples3 calcium sulphate 1 Ca and SO42 Ca +2 and SO4 -23 one Ca for one SO44 formula is CaSO4

45. A few more examples3 calcium sulphate 1 Ca and SO42 Ca +2 and SO4 -23 one Ca for one SO44 formula is CaSO4 4 magnesium hydroxide1 Mg and OH2 Mg is +2 and OH -13 one Mg for two OH4 formula is Mg(OH)2

46. 5 Aluminium carbonate1 Al and CO32 Al +3 and CO3 -23 two Al for three CO34 formula is Al2(CO3)3

47. 5 Aluminium carbonate1 Al and CO32 Al +3 and CO3 -23 two Al for three CO34 formula is Al2(CO3)3 All these examples have had a metal as one part of the compound. If two non metals combine life is a bit more complicated.

48. 5 Aluminium carbonate1 Al and CO32 Al +3 and CO3 -23 two Al for three CO34 formula is Al2(CO3)3 All these examples have had a metal as one part of the compound. If two non metals combine life is a bit more complicated. We often need to just make sure that the numbers are equal for each element. It is easier than that all sounds!

49. Let’s work out some examples1 Silicon dioxide

50. Let’s work out some examples1 Silicon dioxide1,2 Si 4, O 2