Ionic Compounds

At the end of this chapter you should be

able to: Describe the ionic bonding model Use the model to explain the properties of

ionic compounds Explain how ions are produced when metals

and non-metals react Write chemical formulas for ionic compounds Describe the uses of some ionic compounds

Chapter Outcomes

Ionic compounds are made up by the chemical combination of metallic and non-metallic elements.

Most rocks, minerals and gemstones are ionic compounds.

Ceramics, bricks and kitchen crockery are made from clays which contain ionic compounds.

While most of the above are made up of mixtures of different ionic compounds table salt is a pure ionic compound made up of sodium chloride (NaCl)

Ionic Compounds

Think of the properties of rocks, bricks,

crockery and table salt. What properties do they share? Have high melting and boiling temperatures. Are hard but brittle

They also: Do NOT conduct electricity in the solid state They will only conduct electricity if they are

melted or dissolved in water

Properties of Ionic Compounds

The physical properties of ionic compounds

are very different from metals. The structure of ionic compounds must

therefore be very different from those present in metals.

What do we already know about ionic compounds.

Structure of ionic compounds

What do the properties tell us?

From the properties we can conclude:

The forces between the particles are strong. There are no free-moving electrons present,

unlike in metals. There are charged particles present, but in solid

state they are not free to move. When an ionic compound melts, however, the

particles are free to move and the compound will conduct electricity.

Structure

Chemists believe that when metallic and non-

metallic atoms react to form ionic compounds the following steps occur:

Metal atoms lose electrons to non-metallic atoms and become positively charged metal ions.

Non-metal atoms gain electrons from the metal atoms and so become negatively charged non-metal ions.

Large numbers of positive and negative ions formed in this way then combine to form a three-dimensional lattice.

The three dimensional lattice is held together strongly by electrostatic forces of attraction between positive and negative ions. This electrostatic force is called ionic bonding.

The ionic bonding model

How many chlorine ions surround each sodium ion and vice versa?

Using the ionic bonding model to explain the properties of sodium chloride

Ever noticed that when you eat fish and

chips the food may be hot but the salt does not melt.

This is because to melt and ionic solid energy must be provided to allow the ions to break free and move.

NaCl has a high melting temp, this indicates a large amount of energy is needed to reduce the electrostatic attraction between the oppositely charged ions and allow them to move freely.

High Melting Temperature

Unlike metals ionic compounds are not malleable. They break when beaten.

A force can disrupt the strong electrostatic forces holding the lattice in place.

A sodium chloride crystal cannot be scratched easily but if a strong force (a hammer blow) is applied it will shatter.

This is because the layers of ions will move relative to each other due to the force.

During this movement, ions of like charge will become adjacent to each other. Resulting in repulsion

Hardness and Brittleness

Hardness and Brittleness

Figure 6.4 The repulsion between like charges causes this sodium chloride crystal to shatter when it is hit sharply.

In the solid form, ions in sodium chloride are

held in the crystal lattice and are not free to move so cannot conduct electricity.

When the solid melts the ions are free to move.

The movement of these charged particles to an electrode completes an electrical circuit.

In a similar way, when sodium chloride dissolves in water, the ions separate and are free to move towards the opposite charge.

Electrical Conductivity

Conducting Electricity

Metallic atoms have low ionisation energies

and low electronegativities. Non-metallic atoms have high ionisation

energies and low electronegativities. In other words metallic atoms lose electrons

easily and non-metallic atoms gain electrons easily.

Reactions of metals with non-metals

So the metal atoms lose an electron to the

non-metal atoms. In doing so, both atoms will often achieve the

electronic configuration of the nearest noblest gas, which is particularly stable.

Ionic Compounds

When sodium reacts with chlorine: Na atom (1s2 2s2 2p6 3s1) loses an electron to

become 1s2 2s2 2p6 (the same as Neon) Cl atom (1s2 2s2 2p6 3s1 3p5) gains an electron

to become 1s2 2s2 2p6 3s1 3p6 (the same as argon)

Sodium Chloride

Electron Configuration

When sodium and chloride react together

sodium loses an electron and chlorine gains an electron.

Electron Transfer Diagrams

What is happening: Chlorine molecules splitting into

separate chlorine atoms Electrons being transferred from sodium

atoms to chlorine atoms – positively charged sodium and negatively charged chlorine ions are being formed.

Sodium and chloride ions combining to form a three dimensional lattice.

Sodium Chloride

When a non-metal atom gains one or more

electrons, the name of the negative ion ends in –ide.

When a metal atom loses one or more electrons the name of the positive ion is the same as the metal and is always named first.

For example: sodium chloride

Notes:

The charge on an ion is known as its

electrovalency. That is the little positive or negative number to

the top right of a chemical symbol. Sodium has an electrovalency of +1 whilst

chlorine has an electrovalency of -1 Na+1 and Cl-1

Electrovalency

What are the electron configurations for

Magnesium and Oxygen? How many electrons does magnesium need to

lose to get a full outer shell? How many electrons does oxygen need to gain

to get a full outer shell? Draw an electron transfer diagram. What is the electrovalency of a magnesium ion

and an oxide ion?

Magnesium Oxide

What are the electron configurations for Mg

and Cl? So a Mg atom will have a stable outer shell if 2

electrons are removed. A Cl atom only needs to gain one electron. So how can this work?

Magnesium Chloride

MgCl2

Almost every compound in which a metal is

combined with a non-metal displays ionic bonding.

The formulas of simple ionic compounds, such as NaCl and MgCl2 can be predicted from the electron configurations of the atoms.

Chemical Formulas

Elements in groups 1 all have an

electrovalency of +1 (they all have only one electron to lose)

Elements in group 17 all have an electrovalency of -1

What about groups 2 and groups 16? Does this formula work for all atoms?

Electrovalencies

Chemical formulas are part of the language of

chemists. To understand and use this language, you need to follow a number of fules.

Writing Formulas: Rules

Simple Ions The positive ion is place first in the formula, the

negative ion is second. For example, Kf, CuO Positive and negative ions are combined so that

the total number of positive charges is balanced by the total number of negative charges.

For example, CuS, CuCl2, AlCl3 and Al2O3

When there are two or more of a particular ion in a compound, then in the chemical formula the number is written as a subscript after the chemical symbol.For example, Al2O3

Writing Formulas: Rules

Some ions contain more than one atom. These are called polyatomic ions. They include nitrate (NO3

-) and hydroxide (OH-). What else?

If more than one of these ions is used to balance the charge of a compound, then it is placed in brackets with the required number written as a subscript after the brackets.For example Mg(NO3)2 and Al(OH)3

Brackets are not required for the formula of sodium nitrate NaNO3, where there is only one nitrate ion present for each sodium ion.

Polyatomic ions

Some elements form ions with different charges. Iron ions can have a charge of +2 or +3. In this situation you need to specify the

electrovalency when naming the compound. This is done by placing a Roman numeral

representing the electrovalency of the ion immediately after the metal in the name of the compound.

For example Iron(II) chloride contains Fe2+ ions and so the

formula is FeCl2 Iron(III) chloride contains Fe3+ ions and so the

forumla is FeCl3

Different Electrovalencies