Ionic compounds
The Earth’s hydrosphere is the oceans. They consist mainly of water, with
some dissolved compounds. The ionic compounds that are dissolved in sea
water are mainly salts, and make the water taste salty.
Ionic bonding
When metals react with non-metals, electrons are transferred from the metal atoms
to the non-metal atoms, forming ions. The resulting compound is called an ionic
compound.
Here are some examples:
sodium + chlorine → sodium chloride
magnesium + oxygen → magnesium oxide
calcium + chlorine → calcium chloride
In each of these reactions, the metal atoms give electrons to the non-metal atoms,
so that the metal atoms become positive ions and the non-metal atoms become
negative ions.
There is a strong electrostatic force of attraction between these oppositely charged
ions, called an ionic bond.
The animation shows ionic bonds being formed in sodium chloride, magnesium
oxide and calcium chloride:
http://www.bbc.co.uk/schools/gcsebitesize/science/add_ocr/chemicals/ioniccompoun
dsrev1.shtml
Physical properties of ionic compounds
The ions in a compound such as sodium chloride are arranged in a lattice structure.
This regular arrangement results in the formation of a crystal.
The diagram shows part of the crystal lattice of sodium chloride:
Positively charged Na ions, negatively charged Cl ions
This pattern is repeated in all directions, giving a giant three-dimensional lattice
structure in sodium chloride crystals.
Because of the strong electrostatic forces between them, it takes a great deal of
energy to separate the positive and negative ions in a crystal lattice. This means that
ionic compounds have high melting points and boiling points.
Solid ionic compounds do not conduct electricity, because the ions are held firmly
in place. They cannot move to conduct the electric current. But when an ionic
compound melts, the charged ions are fee to move. Molten ionic compounds do
conduct electricity.
When a crystal of an ionic compound dissolves in water, the ions separate. Again,
the ions are free to move, so a solution of an ionic compound in water also conducts
electricity.
Ionic bonding
Sea water contains a number of dissolved salts. When these salts dissolve in water,
the ions separate. Sea water therefore contains a mixture of ions. The most common
ions in sea water are shown in the table:

name of ion

formula and charge

chloride

Cl−

sodium

Na+

sulfate

SO42−
magnesium

Mg2+
calcium

Ca2+



potassium

carbonate

bromide


K+
CO32−

Br−
It is possible to deduce from the table which salts dissolved to form the
mixture of ions:

When sodium chloride dissolves it forms sodium ions and chloride ions.

When magnesium chloride dissolves it forms magnesium and chloride ions.

When magnesium sulfate dissolves if forms magnesium ions and sulfate ions.

When potassium chloride dissolves it forms potassium ions and chloride ions.

When potassium bromide dissolves it forms potassium ions and bromide ions.
You can use the charge on the ions shown in the table to work out the
formulae of the ionic compounds.
Sodium ions each have a single positive charge. Chloride ions each have a
single negative charge. For the charges to cancel out in the neutral salt
sodium chloride, they must be in a ration of 1:1. So the formula of sodium
chloride is NaCl.
Magnesium ions each have two positive charges. Chloride ions each have a
single negative charge. For the charges to cancel out in the neutral salt
magnesium chloride, they must be in a ration of 1:2. So the formula of
magnesium chloride is MgCl2.
Ionic compounds Quiz
Name : ………………………………………………………
1. Which sort of elements join together by ionic bonding?
metals only
non-metals only
metals and non-metals
2. What sort of force holds the ions together in an ionic bond?
electrostatic
magnetic
adhesive
3. Which of these is an ionic compound?
ammonia
hydrogen chloride
potassium chloride
4. What is lost by a metal atom as it forms an ionic bond?
protons
electrons
neutrons
5. Which of these properties do most ionic compounds have?
a high melting point
a low melting point
they are a gas at room temperature.
6. What is the charge on a non-metal ion?
negative
positive
neutral
7. Potassium ions have a 1+ charge, and chloride ions have a 1- charge.
What is the formula of potassium chloride?
K2Cl
KCl
KCl2
8. Magnesium ions have a 2+ charge, while chloride ions have a 1- charge.
What is the formula for magnesium chloride?
Mg2Cl
MgCl
MgCl2
9. Sodium ions have a 1+ charge, and carbonate ions have a 2- charge.
What is the formula of sodium carbonate?
Na2CO3
NaCO3
Na(CO3)2
10. Calcium ions have a 2+ charge, while sulfate ions have a 2- charge.
What is the formula for calcium sulfate?
Ca2SO4
CaSO4
Ca(SO4)2
Ionic compounds - Answers
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1. Which sort of elements join together by ionic bonding?

metals and non-metals
2. What sort of force holds the ions together in an ionic bond?

electrostatic
3. Which of these is an ionic compound?

potassium chloride
4. What is lost by a metal atom as it forms an ionic bond?

electrons
5. Which of these properties do most ionic compounds have?

a high melting point
6. What is the charge on a non-metal ion?

negative
7. Potassium ions have a 1+ charge, and chloride ions have a 1- charge.
What is the formula of potassium chloride?

KCl
8. Magnesium ions have a 2+ charge, while chloride ions have a 1- charge.
What is the formula for magnesium chloride?

MgCl2
9. Sodium ions have a 1+ charge, and carbonate ions have a 2- charge.
What is the formula of sodium carbonate?

Na2CO3
10. Calcium ions have a 2+ charge, while sulfate ions have a 2- charge.
What is the formula for calcium sulfate?

CaSO4