This document provides a table and instructions for writing chemical formulas using electrovalencies. The table lists common ions in the columns for positive and negative charges. The document explains that metals are positively charged while non-metals are negatively charged. It provides examples of writing formulas such as NaCl, CaCl2, MgO, and Al(OH)3 by matching the charges of ions and adding subscripts. Key ions like sulfates, hydroxides, and carbonates are identified that are essential to know for writing chemical formulas.

1. Table of electrovalencies

2.  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+
Na
1+
Sodium
1+
K
Potassium
Li
1+
Lithium
1+
Cu
Copper (I)
1+
Ag
Silver
1+
NH4
Ammonium
2+
Ca
2+
Calcium
2+
Mg
Magnesium
Fe
2+
Iron (II)
2+
Cu
Copper (II)
2+
Mn
Manganese
3+
Al
3+
Aluminium
Cr
3+
Chromium (III)
Fe
3+
Iron (III)
1-
2-
1-
3-
2-
3-
F
O
P
Fluoride
Oxide
Phosphide
Cl
1-
2-
Chloride
Br
1-
N
Sulphate
Nitride
2-
3-
CO3
Bromide
1-
OH
Hydroxide
1-
OCl
3-
SO4
Hypochlorite
1-
PO4
Carbonate
Phosphate
2-
CrO2
Chromate
2-
Cr2O7
Dichromate
2-
HCO3
S2O3
Hydrogencarbonate
Thiosulphate
1-
2-
HSO4
O2
Hydrogensulphate
Peroxide
1-
NO3
P J Jackson
Nitrate
3

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
6

7. Examples
Sodium chloride
Na1+
Cl+ and - both the same so leave as 1
NaCl
Calcium chloride
Ca2+ and O2+ and - both the same so leave as 1
CaO
P J Jackson
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8. Calcium fluoride
Ca2+ and F1Two 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 O2Two pluses and two minuses
So these cancel each other
Therefore
MgO

10. Aluminium hydroxide
Al3+ and OH1Three 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 SO42Need 2 K+ to cancel out 2- of sulphate
K2SO4
P J Jackson
11

12. Ammonium sulphate
NH41+ and SO42One 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 OH3+ and 1so 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
13

14. Copper(II) Phosphide
Cu2+ and P32+ and 32+ to cancel out 2 P3so need 3 Cu
Giving a total of 6+ and 6 Cu3P2
P J Jackson
14

15. Chromium carbonate
Cr3+ and CO32Three 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 HSO423+ and 23+ to cancel out 3 HSO 2so need 2 Al
4
Giving a total of 6+ and 6 Al2(HSO4)3
HSO4- is a radical - put it in brackets
P J Jackson
16

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
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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
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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
19

20. Other examples

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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
 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