C3 – Unit 3

Compounds and Chemical Formulae

Compounds

Compounds are substances formed when atoms of two or more elements join together.

The name of a compound is derived from the names of the elements from which it is formed with a suffix of -ide, -ite, or -ate.

The ending – ide indicates the compound usually contains only the 2 named elements

The ending – ite or – ate indicate the compound contains oxygen as well as the 2 named elements.

 

 

Sodium sulphide contains the elements – sodium and sulphur.

Sodium sulphate contains the elements – sodium, sulphur and oxygen.


Chemical formulae

The chemical formula gives the number of each atom in covalent compounds

CH4           contains 1 atom of carbon and 4 atoms of hydrogen

P2O5          contains 2 atoms of phosphorus and 5 atoms of oxygen

C2H5NH2     contains 2 atoms of carbon, 1 of  nitrogen and 7 of hydrogen

For ionic compounds the formula gives the ratio of each ion in the compound.

Na2O         contains sodium ions and oxygen ions in the ratio 2 : 1

 

a) Formulae from names

The formula can be deduced from the name of the compound, numbers being derived from the prefixes.

prefix

number

mono

1

di

2

tri

3

tetra

4

penta

5

hexa

6

eg.

sulphur trioxide           SO3

carbon tetrachloride     CCl4

 

b) Formulae from Valencies

The valency of an atom is the number of bonds with which it is joined to other atoms.

The valency of an atom is worked out from its group in the periodic table.

fluorine is in group 7 so its valency is 1

phosphorus is in group 5 so its valency is 3

 

The chemical formula of simple substances can be calculated by knowing the valencies of the elements in that substance.

Valencies are cross multiplied

eg. phosphorus fluoride

symbols P F
valencies 3 1
cross over
1 3

so the formula is PF3

If valencies can be cancelled then do so

eg. carbon sulphide

symbols C S
valencies 4 2
cancel 2 1
cross over
1 2

so the formula is CS2 and not C2S4

Formulae for complex ions

These ions, most ending in -te, are found on page 4 of the data book. The charge gives the valency.

 

eg. sodium sulphate

symbols Na SO4
valencies 1 2
cross over 2 1

so the formula is Na2SO4

 

If the complex ion has to be multiplied it must be enclosed in a bracket.

eg. aluminium nitrate

symbols Al NO3
valencies 3 1
cross over
1 3

so the formula is Al(NO3)3

Watch out for ammonium and hydroxide

The hydroxide ion must have brackets when multiplied. It looks right but is actually wrong

 

eg. magnesium hydroxide

symbols Mg OH
valencies 2 1
cross over
1 2

so the formula is Mg(OH)2 and not MgOH2

Formulae from oxidation numbers

Transition metals have their valencies given in the formula (known as the oxidation number)

eg. iron III oxide

symbols Fe O
valencies 3 2
cross over
2  3

so the formula is Fe2Cl3

Formula Mass

The formula mass of a substance is the sum of the masses of all it’s atoms.

It can be calculated from the formula by adding up the relative atomic masses of all the atoms in the compound.

This is best set out using a table.

 

eg. Calculate the formula mass of sodium carbonate

formula      Na2CO3

table

element

RAM

number

total

Na

23

2

46

C

12

1

12

O

16

3

48

formula mass

106

 The unit is the amu but it is usually left dimensionless at this stage.

CFE Outcomes

Level 4

Through gaining an understanding of the structure of atoms and how they join, I can begin to connect the properties of substances with their possible structures.
SCN 4-15a

National 4

Atomic structure and bonding related to properties of materials
Chemical formulae of two-element compounds.

Compounds are substances formed when atoms of two or more elements join together. The name of a compound is derived from the names of the elements from which it is formed with a suffix of -ide, -ite, or -ate. The ratio in which elements combine to form two element compounds can be determined using valency rules. The chemical formula can also be determined from names with prefixes, models or structures.

National 5

Formulae and reaction quantities
Chemical and ionic formulae including group ions.