1. Table of electrovalencies
Simple Formulae
Table of electrovalencies

2. This is a simple way to learn how to write chemical formulae
This is a simple way to learn how to write chemical formulae. There are other ways to work them out but this is simple although it does involve some learning by heart
On a graph page at the back of your practical notebooks draw lines on all the red [or heavier] lines.
Head the columns +1, +2, +3, blank, -1, -2, -3.
 Enter the appropriate radical(ion) and its charge in each square as shown in the next slide

3. 1+ 2+ 3+ 1- 2- 3- Na1+ Ca2+ Al3+ F1- O2- P3- Sodium Calcium Aluminium
Fluoride
Oxide
Phosphide
K1+
Mg2+
Cr3+
Cl1-
SO42-
N3-
Potassium
Magnesium
Chromium (III)
Chloride
Sulphate
Nitride
Li1+
Fe2+
Fe3+
Br1-
CO32-
PO43-
Lithium
Iron (II)
Iron (III)
Bromide
Carbonate
Phosphate
Cu1+
Cu2+
OH1-
CrO22-
Copper (I)
Copper (II)
Hydroxide
Chromate
Ag1+
Mn2+
OCl 1-
Cr2O72-
Silver
Manganese
Hypochlorite
Dichromate
NH41+
HCO31-
S2O32-
Ammonium
Hydrogencarbonate
Thiosulphate
HSO41-
O22-
Hydrogensulphate
Peroxide
NO31-
Nitrate
P J Jackson

4. How to learn the Table Metals are all +
NH4+ and H+ are regarded as metals
Non-metals are always –
Learn +1 and +3
Any other metal = +2
Learn -3 and -2
Any other non-metal is -1

5. Using the Table
This table can be used to work out the formula of most of the compounds one is likely to encounter.

6. Positive ion [metal] goes first
Negative ion [non-metal] or radical goes second
Multiply each ion [radical] by the smallest possible integer to make the total of + and – the same
Place this number as a subscript after each ion. If the ion is complex [i.e. a radical] put it in brackets and put the subscript outside the brackets
P J Jackson

7. Examples
Sodium chloride Na1+ Cl- + and - both the same so leave as 1 NaCl Calcium chloride Ca2+ and O2- CaO
P J Jackson

8. Calcium fluoride Ca2+ and F1- Two pluses and one minus So two F1- are needed to cancel out the 2 pluses of the calcium Therefore CaF2

9. Magnesium oxide Mg2+ and O2- Two pluses and two minuses So these cancel each other Therefore MgO

10. Three pluses and one minus
Aluminium hydroxide
Al3+ and OH1-
Three pluses and one minus
So three OH1- are needed to cancel out the 3 pluses
Therefore Al(OH)3
Notice that the OH is put in brackets as it is a compound ion

11. Aluminium nitride Al3+ and N3- + and - both the same so leave as 1 AlN Potassium sulphate K+ and SO42- Need 2 K+ to cancel out 2- of sulphate K2SO4
P J Jackson

12. One plus and two minuses
Ammonium sulphate
NH41+ and SO42-
One plus and two minuses
So two NH41+ are needed to cancel out the 2 minuses
Therefore (NH4)2SO4
Notice that the ammonium is in brackets as it is a complex ion.

13. Iron(III) hydroxide Fe3+ and OH- 3+ and 1-
so need 3 OH- to cancel out 3+ of iron(III)
OH- is a radical so put it in brackets
Fe(OH)3
Notice that the charges are not normally written in the final form
P J Jackson

14. Copper(II) Phosphide Cu2+ and P3- 2+ and 3- so need 3 Cu2+ to cancel out 2 P3- Giving a total of 6+ and 6 - Cu3P2
P J Jackson

15. Three pluses and two minus
Chromium carbonate
Cr3+ and CO32-
Three pluses and two minus
So two Cr3+ [6+]are needed to cancel out the 3 CO32- [6-]
Therefore Cr2(CO3)3
Notice that as before the CO3 is put in brackets as it is a compound ion

16. Aluminium hydrogensulphate Al3+ and HSO and 2- so need 2 Al3+ to cancel out 3 HSO42- Giving a total of 6+ and 6 - Al2(HSO4)3 HSO4- is a radical - put it in brackets
P J Jackson

17. Essential to know Sulphates [SO42- ] Sulphites [SO32-]
Hydroxides [OH-1]
Carbonates [CO32-]
Hydrogencarbonates [HCO31-]
Nitrates [NO31-]
Of the first 36 elements – excluding the d-block elements – where they exist
P J Jackson

18. Positive ions Lost electrons
One positive charge for each electron lost
If doing s,p,d,f . electron pattern then put in square brackets with charge outside
The pattern inside the bracket will be of the nearest Noble gas [He, Ne or Ar]
Only required for first 20 elements
P J Jackson

19. Negative Ions Gain electrons
One negative charge for each electron gained
If doing s,p,d,f . electron pattern the put in square brackets with charge outside
The pattern inside the bracket will be of the nearest Noble gas [He, Ne or Ar]
Only required for first 20 elements
P J Jackson

20. Other examples Iron (II) nitrate Ammonium sulphate
Sodium hydrogencarbonate
Iron (II) phosphide
Chromium nitride
Calcium Chloride
Aluminium phosphate
Copper (I) oxide
Calcium hydrogencarbonate
Copper (II) dichromate
Magnesium hypochlorite
Potassium dichromate
Sodium thiosulphate
Barium nitrate
Hydrogen peroxide
Iron (III) thiosulphate
Zinc permanganate
Silver carbonate
Iron (II) hydrogensulphate
Aluminium sulphate
Magnesium bromide
Calcium phosphate
Silver nitrate
Manganese peroxide
Calcium carbonate