Table of electrovalencies 2.1.1 Simple Formulae Table of electrovalencies

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

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

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

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

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

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

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

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

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

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

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.

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

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

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

Aluminium hydrogensulphate Al3+ and HSO42- 3+ 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

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

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

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

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