9.06.2 Ionic Compounds

Syllabus

An ionic compound is a giant structure of ions. Ionic compounds are held together by strong electrostatic forces of attraction between oppositely charged ions. These forces act in all directions in the lattice and this is called ionic bonding.
The electron transfer during the formation of an ionic compound can be represented by a dot and cross diagram, eg for Sodium Chloride.
Students should be :
1. draw dot and cross diagrams for ionic compounds formed by metals in Groups 1 and 2 with non-metals in Groups 6 and 7.
2. able to deduce a compound is ionic from a diagram of its structure in one of the specified forms
3. able to describe the limitations of using dot and cross, ball and stick, two and three-dimensional diagrams to represent a giant ionic structure
4. able to work out the empirical formula of an ionic compound from a given model or diagram that shows the ions in the structure.
5. familiar with the structure of Sodium Chloride but do not need to know the structures of other ionic compounds.

What does this mean?

If one element loses an electron to become positive.......

.....another must gain to become negative.

Positive and negative ions will attract each other electrostatically - this is an Ionic Bond.

When chemists draw electrons they tend to use dots.

When drawing dot cross diagrams of the ionic bonding in a substance dots are used to show electrons in one element, crosses in the other.

In the example above, we knew Sodium atoms would lose one electron to attain the electronic structure (2,8), so the dot-cross diagram shows two full shells in the ion formed and a 1+ charge (examiners sometimes only want the outside shell).

We knew the Chlorine atom needed to gain one electron to attain (2,8,8) and the dot-cross diagram makes it clear that it gained its extra electron from the Sodium atom.

A dot-cross diagram may sometimes show the electron being moved (using an arrow) and must always show the charges of the ions formed.

In the example above, we knew Calcium atoms would lose two electrons to attain the electronic structure (2,8,8), so the dot-cross diagram shows three full shells in the ion formed and a 2+ charge.

We knew the Oxygen atom needed to gain two electrons to attain (2,8,8) and the dot-cross diagram makes it clear that it gained its extra electrons from the Calcium atom.

Some more complicated examples.

If the elements match up well, so that the metal needs to lose the same amount of electrons as the non-metal needs to gain, the dot-cross diagram is easy to draw because it will only contain two ions.

This always happens between Group 1 and Group 7, and between Group 2 and Group 6.

But if the elements match up less well it will be necessary to have more than two ions involved.

In the example below, Calcium atoms need to lose two electrons.

But Chloride ions are full when they have accepted one.

So two Chloride ions were needed and the formula of Calcium Chloride is CaCl2.

In the next example below, Sodium atoms need to lose one electron.

But Oxide ions are full when they have accepted two .

So two Sodium ions were needed and the formula of Sodium Oxide is Na2O

Lattice Structure

We saw the Dot-Cross diagram for Sodium Chloride (NaCl)

This explains what happens with electrons but it's a poor model of the substance formed.

The positive Sodium ions don't just attract one negative ion each - they get surrounded by negative ions (Anions).

And the negative anions get surrounded by positive ions (cations)

So you will see the below image of a Sodium Chloride lattice a lot.

It's a 3D model representing a small part of a giant structure - even the smallest salt grain has billions of billions of ions, but all in this arrangement.

Except, that they are not separated by stick-like bonds. The ions are packed tightly, more like the model below.

Which is easy enough to see in 2 dimensions but in 3D it looks like...

Which makes it much harder to see what it is like inside.

We need to be able to write a formula for a given lattice.

What is the formula of Lithium Chloride?

Looking at the entire model, there are a total of 14 Cl- ions and 13 Li+ ions.

The formula isn't Li13Cl14 because this is only a section of the whole crystal.

The ratio between the ions is 13:14 very close to 1:1

If we look at only two layers we can see the ratio is exactly 9:9 , or 1:1

So the Formula is Li1Cl1 - although we would only write LiCl

Other Formulas

The whole structure has a Cd:Cl ratio of 6:14 - which is almost 1:2.

If we look at two layers again the ratio is 4:9 - which is even closer to 1:2

So, our best guess of a formula would be CdCl2

You can assume that the ratio will be very close on an exam and very easy to read because some lattices have shapes where working out the formula is much more of a challenge.

Don't be afraid to suggest a formula of MX if you count 20 M ions and 19 X ions.

Or a formula of M2X if you count 20 M ions and 11 X ions