Chemical Formulae
Throughout Standard Grade Chemistry, you will be looking at a variety of different
topics, ranging from plastic to cow poo! But at the heart of it, Chemistry is all about
different atoms joining together.
Atoms wish to be like the Nobel Gases (Group 8 or 0), because they have a Full Outer
Energy level. e.g. Neon 2, 8
Other atoms can achieve a full outer energy level by losing, gaining or sharing electrons
Group 1 e.g. Na 2,8,1
Gives away 1 electron to another atom, so forms ONE BOND
Group 2 e.g. Mg 2,8,2
Gives away 2 electrons to other atoms, so forms TWO BONDS
Group 3 e.g. Al 2,8,3
Gives away 3 electrons to other atoms, so forms THREE BONDS
Group 4 e.g. C 2,4
Has a half-filled energy level, so they don’t gain or lose. Instead they share electrons.
They need a share of 4 electrons, so forms FOUR BONDS.
Group 5 e.g. N 2,5
Need a share of 3 electrons or needs to gain 3, so forms THREE BONDS
Group 6 e.g. O 2,6
Need a share of 2 electrons or needs to gain 2, so forms TWO BONDS
Group 7 e.g. F 2,7
Needs a share of 1 electron or needs to gain 1, so forms ONE BOND
Group 8 e.g. Ar 2,8,8
Do not need to lose, gain or share electrons, so form NO BONDS!!!
The no. of bonds that an atom forms is called its Valency. This is because the bonding
of atoms involves its outer electrons, which are formally known as their valence
electrons
Chemical Formula
The no. of each atom in a molecule can be expressed by its chemical formula
e.g. Water contains 2 atoms of hydrogen and one atom of oxygen, so its formula is H2O
Using the valencies of elements, enables us to work out how many of each atom are
contained in a molecule. Compounds which end in “ide” contain 2 different elements.
Examples
Carbon Hydride
Symbol
C
H
Valency
4
1
Swap
1
4
Formula
CH4
Magnesium Oxide
Symbol
Mg
O
Valency
2
2
Swap
2
2
Formula
Mg2O2
which can be simplified to MgO
Roman Numerals
The valency of transition metal elements can be different, depending on what they are
bonded to e.g. Iron can have a valency of 1, 2 or 3!
The valency or the element is indicated in the compound name
e.g. Iron (I) Chloride, Iron (II) Chloride or Iron (III) Chloride.
IV would mean a valency of 4 and V would mean a valency of 5
Iron(I) Chloride
Symbol
Fe
Cl
Valency
1
1
Swap
1
1
Formula
FeCl
Iron(II) Chloride
Symbol
Fe
Cl
Valency
2
1
Swap
1
2
Formula
FeCl2
Group Ions
In some cases, we can have a number of atoms joining together to form a group ion
e.g. A sulphur and 4 oxygens combine to form a sulphate ion SO4 (see p4 of data book).
These group ions have a charge.
Since the charge is caused by gaining or losing electrons, then
the size of the charge also tells us the number of bonds the ions form.
SO42- would form 2 bonds, whereas NO3- would form 1 bond
When working out the formula of compounds containing these groups, it is important
that we do not change the no. of atoms in the group.
e.g. SO4 is sulphate but SO3 is sulphite
If a compound ends in “ate” or “ite” you must find the group ion in the data book
We treat the group like it is in a bubble, we do not change its formula
Sodium Nitrate
Symbol
Na
NO3
NO3
Valency
1
1
Swap
1
1
Formula
NaNO3
Aluminium Sulphate
Symbol
Al
SO4
Valency
3
2
Swap
2
3
Formula
Al2
SO4
3
Its looks silly having it in a bubble, so if we have more than one of the ions then
we use brackets e.g. Al2(SO4)3
Ionic Formula
Ionic compounds are made up of charged particles. It is sometimes necessary to show
the charges on the ions which make up an ionic compound.
Metal ions have a positive charge, since they are formed when a metal atom loses
negatively charged electrons.
Non-metal ions have a negative charge since they are formed when a non-metal atom
gains negatively charged electrons.
To show the ionic formula of a compound, first we must establish its chemical formula.
Sodium Chloride
Symbol
Na
Cl
Valency
1
1
Swap
1
1
NaCl
Formula
We then put in the charge for each ion.
Sodium is a metal, so has a positive charge.
It has a valency of 1, therefore it will have a 1+ charge i.e. Na+
Chlorine is a non-metal, so it has a negative charge.
It has a valency of 1, therefore it will have a 1- charge i.e. ClTherefore the ionic formula of sodium chloride is Na+ClNote that the charges balance each other out, so overall the compound is neutral.
Even though the charges balance each other out, we must still show them.
Iron(II) Chloride
Symbol
Fe
Cl
Valency
2
1
Swap
1
2
Formula
FeCl2
The roman numeral tells us that iron has a valency of 2, therefore it has a 2+ charge.
Chloride ions as we saw above, have a 1- charge.
So the ionic formula for iron(II) chloride would be Fe2+ (Cl-)2
Note: If we have more than one ion, we must put the symbol and charge in
brackets. The 2 chloride ions with their 1- charges, balances the 2+ on the iron.
Meaningful Names
Some compounds do not follow the valency rules, but working out the chemical formula
of these compounds is much easier than using the valency.
We can work out the formula by looking at the name of the compound.
Carbon Monoxide
DiNitrogen Oxide
Phosphorus TriChloride
Carbon TetraChloride
Nickel PentaIodide
Uranium HexaFluoride
The prefixes tell us the no. of atoms present.
Mono – 1, Di – 2, Tri – 3, Tetra – 4, Penta – 5, Hexa – 6
If a compound name contains one of these prefixes, then valency does not apply
Carbon Monoxide
The name tells us there is 1 Oxygen and if no prefix is given for the other element, we
assume that there is 1 atom. i.e. CO
DiNitrogen Oxide
The name tells use there are 2 Nitrogens and there is no prefix on the oxide, so we
assume that there is 1 oxygen i.e. N2O
Work out the rest for yourself.
Work out the formula for the following compounds:
Calcium Fluoride
Nitrogen Hydride
Hydrogen Sulphide
Lithium Nitrate
Sodium Carbonate
Magnesium Sulphate
Silver(I) Iodide
Copper(II) Chloride
Iron(II) Nitrate
Work out the ionic formula for the following compounds:
Potassium Chloride
Sodium Sulphide
Aluminium Oxide
Calcium Hydroxide
Iron(III) Sulphite
Potassium Phosphate