Valency

In the previous topic, 'Bonding, structure and properties', you came across various types of bonding that allowed atoms of different elements to react together and achieve a stable electron arrangement, namely Covalent bonding and Ionic bonding.

In both of these types of bonding, the atoms needed to lose or gain a certain number of electrons in order completely fill their outer shell. This number is called the Valency and is useful in determining the chemical formula of the compound that this atom will form when it reacts with another atom.

The valency of an atom is also called its Combining power.

e.g. The non-metal oxygen has an electron arrangement of 2,6

This means that oxygen needs to gain two electrons to achieve a full outer shell.

The valency of oxygen atom is therefore 2

e.g. The metal sodium has an electron arrangement of 2,8,1

This means that sodium needs to lose one electron to achieve a full outer shell.

The valency sodium is therefore 1

You learned in the 'Structure of the atom' topic, that atoms in the same group of the periodic table have similar chemical properties because they have the same number of electrons in their outer energy level. This means that they need to lose/gain the same number of electrons as each other to achieve a stable elecron arrangement. This means that they will have the same valency number as each other. These are as follows :

 
Covalent bonding and valency

When two atoms combine, either by a covalent bond or an ionic bond, the ratio of atoms in compound is determined by the combining power or valency of each atom.

The valency for non-metal atoms tells us the number of covalent bonds that an atom needs to form to achieve a stable electron arrangement.

Valency card pictures help you decide how many covalent bonds need to be made, some of which are shown below :

           

For example, hydrogen atoms (group 1), need a further electron in their outer shell to achieve a stable electron arrangement, and therefore have a valency of 1. This means that each hydrogen atom needs to make one covalent bond with another non-metal atom.
It is possible for two hydrogen atoms to form a single covalent bond together, forming a diatomic molecule of hydrogen as shown below.

Hydrogen atoms				Hydrogen molecule

For example nitrogen (group 5, electron arrangement : 2,5 and valency=3), needs to form three covalent bonds to achieve a stable electron arrangement. It can do this with three hydrogen atoms as shown below :

Nitrogen atom		Hydrogen atoms						Nitrogen hydride molecule

This method also works for other covalent compounds and diatomic molecules, as shown below :

The chemical formula of a covalent compound can be simplified by using the following technique :

The table below shows some examples of this method :

	Hydrogen molecule	Hydrogen oxide	Carbon hydride	Carbon oxide	Phosphorus oxide
Symbols	H     H	H     O	C     H	C     O	P     O
Valency	1     1	1     2	4     1	4     2	3     2
Cross over	1     1	2     1	1     4	2     4	2     3
Simplest ratio	1     1	2     1	1     4	1     2	2     3
Formula	HH or H2	H2O	CH4	CO2	P2O3

You can quickly test your knowledge of the above information.

 
Ionic bonding and valency

When writing chemical formulae for ionic compounds, the valency method above also holds true, with the exception that the first three rows of group 4 of the periodic table do not usually form ions, and so cannot be given an ionic valency.

When an ionic bond is formed between two atoms, a metal loses one or more electrons and a non-metal gains one or more electrons in order to give both atoms a stable electron arrangement. When this happens, two ions are formed - one with a positive charge (normally a metal) and one with a negative charge (a non-metal).

In order to form an ionic compound, the compound must have no overall charge associated with it. In other words there must be the same amount of positive charges present as negative charges. In this way, the chemical formula for an ionic compound can be deduced. This is shown below alongside the valency method :

Balancing Charges Sodium chloride Magnesium sulphide Aluminium oxide Symbol Na     Cl Mg     S Al     O Charge on ion formed 1+     1- 2+     2- 3+     2- No. of each ion required to balance charges 1     1 1     1 2     3 Chemical formula NaCl MgS Al2O3

Valency method Sodium chloride Magnesium sulphide Aluminium oxide Symbol Na     Cl Mg     S Al     O Valency 1     1 2     2 3     2 Cross over 1     1 2     2 2     3 Simplest ratio 1     1 1     1 2     3 Chemical formula NaCl MgS Al2O3

Often, ions exist that are not single charged atoms, but several atoms that are bonded together with an overall charge associated with them. The ions are called group ions and two examples are the sulphate ion and the nitrate (others can be found in the data booklet).
The formula of these ions are SO₄^2- and NO₃^- respectively.

The charge associated with these group ions can be classed as the valency when working out chemical formula.

e.g. The compound sodium sulphate would have the formula Na₂SO₄ and the compound calcium nitrate would have the formula Ca(NO₃)₂.

Notice that you must put brackets around group ions if there is more than one group of them present in a chemical formula.

You can quickly test your knowledge of the above information.

 
Determining chemical formulae from the chemical name

Often compounds exist that do not follow the simple valency rule for working out the chemical formula. The names of these compounds usually give an indication of the elements present and the quantity of each in the compound.

Carbon monoxide and carbon dioxide each contain carbon joined to oxygen. The prefixes 'mon' (or mono) and 'di' are used to show how many atoms of oxygen are present. The table shows show what the prefixes mean :

• Carbon monoxide means carbon and one atom of oxygen
• Carbon dioxide means carbon and two atoms of oxygen

(No valency numbers are used to write formulae with prefixes.)

Another type of specially named compound involves elements that can have more than one valency number. This is true of many of the transistion metals.

These elements have their valency given as Roman numerals after the symbol for that element.

e.g. Copper (II) Chloride tells us that the element copper has the valency of 2 associated with it. This would make the chemical formula CuCl₂.

e.g. Copper (I) Chloride tells us that the element copper has the valency of 1 associated with it. This would make the chemical formula CuCl.

Roman numeral :	I	II	III	IV	V	VI
Valency :	1	2	3	4	5	6

The commonly used roman numerals for valencies are :

The roman numeral after an element also tells you the size of the charge that exist on that atom.

You can quickly test your knowledge of the above information.

 
Balanced equations

When a reaction occurs, we can write a word equation that explains the reaction that has taken place. You learned how to do this in the 'Substances' topic at the beginning of this unit. Remember that the reactants are written on the left of the arrow (the beginning of the reaction) and the products are written on the right of the arrow (the end of the reaction).

e.g. When magnesium metal was burned in air (a source of oxygen), the reaction gave a bright glow and a new compound called magnesium oxide was formed.

The word equation for this reaction is :

magnesium + oxygen magnesium oxide

Since you have now learned how to write the chemical formula of compounds and elements, you can replace these words with the chemical formulae of each of the substances.

• The element magnesium is written as Mg
• The element oxygen (a diatomic molecule) is written as O₂
• The compound magnesium oxide is written as MgO (both magnesium and oxygen have a valency of two and when these valencies are 'crossed over' and expressed as the simplest ratio, the formula gives MgO).

The equation now looks like this :

Mg + O₂ MgO

This equation is called an unbalanced equation, as the number of each type of atom on the left of the arrow is not the same as the number of each type of atom on the right of the arrow.
In this example, the amount of oxygen on the left hand side of the arrow is not the same as the amount of oxygen on the right hand side of the arrow. We need to form 2 'lots' of MgO so as to have two oxygens on the right hand side of the equation.

The equation now looks like this :

Mg + O₂ 2MgO

Again this equation is still not entirely accurate as we now have two magnesium on the right hand side of the arrow, but only one on the left hand side . The reaction must need another magnesium at the beginning of the reaction (on the left hand side of the arrow).

The equation now looks like this :

2Mg + O₂ 2MgO

This is what we call a balanced equation. i.e. the number of atoms of each element on the left hand side of the arrow is the same as the number of atoms of each element on the right hand side of the arrow.

In general, to write a balanced chemical equation :

1. Write a word equation
2. Put the chemical formula for each reactant and product in the reaction (remembering about diatomic elements)
3. Count the number of each type of atom on each side of arrow
4. Multiply the necessary formulae to balance each type of atom
5. Repeat the previous 2 steps until the number of each type of atom on each side of the arrow is the same

 
Formula mass

The formula mass of an element or compound is the sum of the mass of all of the atoms that it contains.

In the 'Structure of the atom' topic, you learned that atoms in the same element can have different masses due to different numbers of neutrons and these atoms were called isotopes.
When we work out the formula mass of an element or compound, we take into consideration the mass of the isotopes and the percentage of each. This is done by simply using the Relative Atomic Mass of the elements involved. These values are given in the data book.

In the element Helium (chemical formula - He), the formula mass is simply the relative atomic mass of one atom of helium. This is given in the data book as 4. Therefore the formula mass of Helium is four.

If we consider the element Hydrogen, we must remember that it is a diatomic element and the formula mass is therefore 2 x 1 = 2.

Another example is the molecule Hydrogen chloride. This has one atom of hydrogen (relative atomic mass = 1) and one atom of chlorine (relative atomic mass of 35.5). This give the formula mass of hydrogen chloride as 36.5

Work out the formula mass of the compound ammonium phosphate.

Firstly write the chemical formula.

	ammonium phosphate
Symbols :	NH4+ PO43-
Valency :	1       3
Cross over valencies :	3       1
Simplest ratio :	3       1
Formula :	(NH4)3PO4

Next work add up the relative atomic masses of all of the elements involved.

3 x N	=	3 x 14	=	42
12 x H	=	12 x 1	=	12
1 x P	=	1 x 31	=	31
4 x O	=	4 x 16	=	64
Formula mass =				149

The formula mass of the compound ammonium phosphate is 149.

 
The mole

One mole of a substance is equal to the formula mass expressed in grams.
It is also called the gram formula mass.

In the previous worked example for the compound ammonium phosphate, the formula mass was calculated to be 149. This means that one mole of ammonium phosphate has the mass of 149g.

The units for the mole is mol.

When working out calculations involving formula masses and moles, you can use the triangle on the right.

What is the mass of 2 moles of hydrogen chloride ?

Firstly write the chemical formula.

Symbols :	H       Cl
Valency :	1       1
Cross over valencies :	1       1
Simplest ratio :	1       1
Formula :	HCl

Next work add up the relative atomic masses of all of the elements involved.

1 x H	=	1 x 1	=	1
1 x Cl	=	1 x 35.5	=	35.5
Formula mass =				36.5

The formula mass of the compound hydrogen chloride is 36.5.

This means that one mole of hydrogen chloride has a mass of 36.5g
Therefore two moles of hydrogen chloride has a mass of (2 x 36.5) = 73g

How many moles of Copper (II) Carbonate are there in 494g of the substance ?.

Firstly write the chemical formula.

Symbols :	Cu2+ CO32-
Valency :	2       2
Cross over valencies :	1       1
Simplest ratio :	1       1
Formula :	CuCO3

Next work add up the relative atomic masses of all of the elements involved.

1 x Cu	=	1 x 63.5	=	63.5
1 x C	=	1 x 12	=	12
3 x O	=	1 x 16	=	48
Formula mass =				123.5

The formula mass of the compound copper (II) carbonate is 123.5.

This means that one mole of copper (II) carbonate has a mass of 123.5g
Therefore 494g contains (494/123.5) = 4 moles of copper (II) carbonate (or 4 mol).

You can quickly test your knowledge of the above information.

New words and their meanings

Valency / Combining power - Gives information about how many single bonds an atom (or group of atoms) needs to form to achieve a stable electron arrangement. It is also the number of electrons that an atom needs to lose or gain in order to achieve a stable electron arrangement.

Group ions - A group of atoms that have an overall charge (e.g. the ammonium ion - NH₄⁺). These ions are listed in the data booklet.

Word equation - A way of representing a chemical reaction using the names of the reactants and the products.

Unbalanced equation - A way of representing a chemical reaction using the chemical formulae of the reactants and the products.

Balanced equation - This is obtained when the numbers of each type of atom on the left a reaction arrow (the reactants) is the same as the number of each type of atom on the right of the reaction arrow (products) when chemical formulae are written.

Formula mass - The sum of the relative atomic masses of all the atoms in an element or compound.

Mole - The formula mass of a substance expressed in grams.

Gram formula mass - The formula mass of a substance expressed in grams (another term for 1 mole of a substance).