2. Chapter 6
Ionic Bonding
6.1 The Stable Electronic Configuration of
a Noble Gas
6.2 Forming Ions
6.3 Ionic Bond: Transferring Electrons
6.4 Chemical Formulae of Ionic Compounds
6.5 Structure and Physical Properties of Ionic
Compounds
2
3. 6.1
The Stable Electronic Configuration
of a Noble Gas
Learning Outcome
At the end of this section, you should be able to:
• describe the stable electronic configuration of
a noble gas.
3
4. 6.1
The Stable Electronic Configuration
of a Noble Gas
What are Noble Gases?
• Elements that belong to Group 0 of the
Periodic Table
• Examples: He, Ne, Ar, Kr and Rn
• Atoms of noble gases are stable and
unreactive.
• They exist in nature as single atoms.
4
5. 6.1
The Stable Electronic Configuration
of a Noble Gas
What is the Noble Gas Structure?
• Noble gases have full or complete outer shells.
Helium has a duplet configuration
(2 outer electrons).
All other noble gases have an octet configuration
(8 outer electrons).
5
6. 6.1
The Stable Electronic Configuration
of a Noble Gas
Why Do Atoms React?
• Atoms of most other elements are reactive
because they do not have the noble gas structure
(i.e. their outer shells are not fully-filled).
• Atoms of these elements lose, gain or share
outer electrons to attain the noble gas
configuration and form compounds.
6
7. 6.1
The Stable Electronic Configuration
of a Noble Gas
Chemical Bonding
Atoms gain or lose
electrons to attain
noble gas configuration
Ionic bonding
Atoms share electrons
to attain noble gas
configuration
Covalent bonding
7
8. Chapter 6
Ionic Bonding
6.1 The Stable Electronic Configuration of a
Noble Gas
6.2 Forming Ions
6.3 Ionic Bond: Transferring Electrons
6.4 Chemical Formulae of Ionic Compounds
6.5 Structure and Physical Properties of Ionic
Compounds
8
9. 6.2 Forming Ions
Learning Outcome
At the end of this section, you should be able to:
• describe the formation of positive ions (cations)
and negative ions (anions) to achieve the noble
gas configuration.
9
10. 6.2 Forming Ions
What is an Ion?
Recall:
Atoms have an equal number of
protons and electrons. They are
electrically neutral.
• An atom loses or gains electrons to form ions.
• Ions are charged particles.
No. of electrons ≠ No. of protons
10
11. 6.2 Forming Ions
What is an Ion?
•
•
Positively-charged ions are called cations.
•
URL
Ions can be positively- or negatively-charged.
Negatively-charged ions are called anions.
11
12. 6.2 Forming Ions
Formation of Cations
•
Atoms of metals lose electrons to form
positively-charged ions called cations.
•In this way, they achieve the noble gas
configuration.
12
13. 6.2 Forming Ions
Example 1: Formation of sodium (Na+) ion
Na atom
Electronic configuration: 2, 8, 1
Number of protons = 11
Number of electrons = 11
The Na atom loses one outer electron to form the
Na+ ion. Why?
To achieve stable octet (noble gas) configuration.
Neon (2, 8)
13
14. 6.2 Forming Ions
Example 1: Formation of sodium (Na+) ion
2, 8
2, 8, 1
sodium atom loses one
outer electron
Na atom: 11p, 12n, 11e
Charge = 11p + 11e
= (+11) + (–11)
=0
Neutral
Na atom
+
Na+ ion: 11p, 12n, 10e
Charge = 11p + 10e
= (+11) + (–10)
= +1
Positively-charged
Na+ ion
14
15. 6.2 Forming Ions
Example 2: Formation of calcium (Ca2+) ion
2, 8, 8
2, 8, 8, 2
calcium atom loses
two outer electrons
Ca atom: 20p, 20n, 20e
Charge = 20p + 20e
= 20(+1) + 20(–1)
= (+20) + (–20) = 0
Neutral
Ca atom
2+
Ca2+ ion: 20p, 20n, 18e
Charge = 20p + 18e
= 20(+1) + 18(–1)
= (+20) + (–18) = +2
Positively-charged
Ca2+ ion
15
16. 6.2 Forming Ions
Common Cations and Their Charges
Metal
Ion
Formula of ion
sodium
sodium ion
Na+
potassium
potassium ion
K+
calcium
calcium ion
Ca2+
magnesium
magnesium ion
Mg2+
aluminium
aluminium ion
Al3+
16
17. 6.2 Forming Ions
Formation of Anions
• Atoms of non-metals gain electrons to form
negatively-charged ions called anions.
•In this way, they achieve the noble gas
configuration.
17
18. 6.2 Forming Ions
Example 1: Formation of chloride (Cl–) ion
Cl atom
Electronic configuration: 2, 8, 7
Number of protons = 17
Number of electrons = 17
What happens in the formation of a chloride ion?
The chlorine atom gains one electron in its outer shell to
achieve a stable octet (noble gas) configuration.
Argon (2, 8, 8)
18
19. 6.2 Forming Ions
Example 1: Formation of chloride (Cl–) ion
2, 8, 7
2, 8, 8
chlorine atom gains
one electron
Cl atom: 17p, 18n, 17e
Charge = 17p + 17e
= (+17) + (–17)
=0
Neutral
Cl atom
Cl– ion: 17p, 18n, 18e
Charge = 17p + 18e
= (+17) + (–18)
= –1
Negatively charged
Cl– ion
19
20. 6.2 Forming Ions
Example 2: Formation of oxide (O2–) ion
2, 6
oxygen atom gains
two electrons
2, 8
2–
O atom: 8p, 8n, 8e
O2– ion: 8p, 8n, 10e
Charge = 8p + 8e
= (+8) + (–8)
=0
Charge = 8p + 10e
= (+8) + (–10)
= –2
Neutral
O atom
Negatively charged
O2– ion
20
21. 6.2 Forming Ions
Common Anions and Their Charges
Non-metal
Ion
Formula of ion
chlorine
chloride ion
Cl–
bromine
bromide ion
Br–
oxygen
oxide ion
O2–
sulfur
sulfide ion
S2–
21
22. Why do metals lose electrons to
form positive ions (cations) but
non-metals gain electrons to form
negative ions (anions)?
22
23. Chapter 6
Ionic Bonding
6.1 The Stable Electronic Configuration of a
Noble Gas
6.2 Forming Ions
6.3 Ionic Bond: Transferring Electrons
6.4 Chemical Formulae of Ionic Compounds
6.5 Structure and Physical Properties of Ionic
Compounds
23
24. 6.3 Ionic Bond: Transferring Electrons
Learning Outcome
At the end of this section, you should be able to:
• describe how an ionic bonds are formed
between metals and non-metals.
24
25. 6.3 Ionic Bond: Transferring Electrons
Ionic Bonding
• Ionic bonds are formed between metals and
non-metals.
Examples:
Group VII: Fluorine, chlorine
Group VI: Oxygen, sulfur
Examples:
Group I: Sodium, potassium
Group II: Magnesium, calcium
• This is done through the transfer of electron(s)
from metals to non-metals.
25
26. 6.3 Ionic Bond: Transferring Electrons
Ionic Bonding
Metallic atom
loses electron(s)
Non-metallic atom
gains electron(s)
Positive ion
Negative ion
(cation)
(anion)
electrostatic forces of
attraction
(hold oppositely charged
ions together)
26
27. 6.3 Ionic Bond: Transferring Electrons
Formation of Ionic Compound
Example 1: Sodium chloride
Step 1: Formation of Positive Ions
Each sodium atom (Na) loses its single outer electron
to form a positively-charged sodium ion (Na+).
Na
2, 8, 1
Na+ + e−
2, 8
27
28. 6.3 Ionic Bond: Transferring Electrons
Step 2: Formation of Negative Ions
Each chlorine atom gains an electron from a
sodium atom to form a negatively-charged
chloride ion (Cl−).
Cl
2, 8, 7
+
e−
Cl –
2, 8, 8
28
29. 6.3 Ionic Bond: Transferring Electrons
Step 3: Formation of Ionic Bonds
Loses
one electron
Sodium atom
2, 8, 1
Gains
one electron
Chlorine atom
2, 8, 7
Electrostatic forces
of attraction
Sodium ion Chloride ion
2, 8, 8
2, 8
Sodium and chlorine react in the ratio of 1 : 1 to form
sodium chloride (NaCl).
URL
29
30. 6.3 Ionic Bond: Transferring Electrons
Example 2: Magnesium chloride
Magnesium
atom loses two
electrons.
Chlorine atoms
gain one electron each.
Chloride ion Magnesium Chloride ion
2, 8, 8
2, 8, 8
ion
2, 8
Magnesium reacts with chlorine in the ratio of 1 : 2
to form magnesium chloride (MgCl2).
30
31. Chapter 6
Ionic Bonding
6.1 The Stable Electronic Configuration of a
Noble Gas
6.2 Forming Ions
6.3 Ionic Bond: Transferring Electrons
6.4 Chemical Formulae of Ionic Compounds
6.5 Structure and Physical Properties of Ionic
Compounds
31
32. 6.4 Chemical Formulae of Ionic Compounds
Learning Outcome
At the end of this section, you should be able to:
• deduce the chemical formula of an ionic compound
from the charges on the ions and vice versa.
32
33. 6.4 Chemical Formulae of Ionic Compounds
Chemical Formulae of Ionic Compounds
• The formula of an ionic compound is
constructed by balancing the charges on the
positive and negative ions.
• All the positive charges must equal all the
negative charges in an ionic compound.
33
34. 6.4 Chemical Formulae of Ionic Compounds
Example: Magnesium oxide
Oxygen forms
O2− ions.
Magnesium forms
Mg2+ ions.
Mg2+
O2−
Charge: +2
Charge: −2
Since 1 × (+2 charge) balances out 1 × (−2 charge),
The formula is MgO.
34
35. 6.4 Chemical Formulae of Ionic Compounds
Example: Copper(II) hydroxide
Copper ion
Hydroxide ion
Cu2+
Charge: +2
OH−
Charge: −1
To balance the charges, multiply the smaller charge (−1) by
2 to make it equal to +2.
Since 1 × (+2 charge) balances out 2 × (−1 charge),
The formula is Cu(OH)2.
35
36. 6.4 Chemical Formulae of Ionic Compounds
Example 1
Write the chemical formula of aluminium oxide.
oxide ion
aluminium ion
Al 3 +
O 2−
Charge: −2
Charge: +3
Al2O3
Therefore, the formula is Al2O3.
36
37. 6.4 Chemical Formulae of Ionic Compounds
Example 2
Write the chemical formula of calcium carbonate.
calcium ion
Ca 2 +
carbonate ion
CO3 2 −
Charge: +3
Ca2(CO3)2
Charge: −2
CaCO3
Since ‘2’ is a common factor, it can be removed.
Therefore, the formula is CaCO3.
URL
37
38. Chapter 6
Ionic Bonding
6.1 The Stable Electronic Configuration of a
Noble Gas
6.2 Forming Ions
6.3 Ionic Bond: Transferring Electrons
6.4 Chemical Formulae of Ionic Compounds
6.5 Structure and Physical Properties of
Ionic Compounds
38
39. 6.5
Structure and Physical Properties of
Ionic Compounds
Learning Outcomes
At the end of this section, you should be able to:
• state that ionic compounds form giant lattice
structures;
• deduce the formulae of ionic compounds from their
lattice structures;
• relate the physical properties of ionic compounds to
their lattice structures.
39
40. 6.5
Structure and Physical Properties of
Ionic Compounds
Structure of Ionic Compounds
Ionic compounds form giant ionic structures.
Also known as giant lattice structures or
crystal lattices
Consist of an endlessly repeating three-dimensional
lattice of positive and negative ions
Ions are closely packed, arranged in an orderly
manner and held in place by ionic bonds
40
41. 6.5
Structure and Physical Properties of
Ionic Compounds
Structure of NaCl
Sodium chloride
crystal
Three-dimensional
arrangement of sodium
ions and chloride ions
Sodium ions and chloride ions
alternate with each other.
41
42. 6.5
Structure and Physical Properties of
Ionic Compounds
Structure of NaCl
Strong forces of attraction
between ions in crystal
lattice
Na+
Cl
–
Na+
Cl–
A large amount of energy
is required to overcome
these forces of attraction
between ions.
Cl– Na+
Na+
Cl–
Na
+
Na+ Cl–
Cl–
Na+
Na+ Cl–
Cl–
Na+
Cl
–
Cl– Na+
Na+ Cl–
Na+
Cl
–
Na
+
Na+ Cl–
Cl–
Cl– Na+
Na
Na+ Cl–
+
Na+
Cl–
Cl–
Na+
Na+
Cl–
Na+ Cl–
Na+
Cl–
Cl–
Na+ Cl–
Na+ Cl–
Na+
42
43. 6.5
Structure and Physical Properties of
Ionic Compounds
Structure of NaCl
Cl− ion
Each chloride ion is
surrounded by six sodium
ions.
Na+ ion
Each sodium ion is
surrounded by six chloride
ions.
The ratio of sodium ions to chloride ions is 1 : 1.
Hence, the formula unit of sodium chloride is NaCl.
43
44. 6.5
Structure and Physical Properties of
Ionic Compounds
Melting and Boiling Points of
Ionic Compounds
• High melting and boiling
points
Na+
Cl
–
Na+
• Non-volatile
Cl–
Cl– Na+
Na+
Cl–
Na
+
Na+ Cl–
• Exist as solids at room
temperature
Cl–
Na+
Na+ Cl–
Cl–
Na+
Cl
–
Cl– Na+
Na+ Cl–
Na+
Cl–
Na
+
Na+ Cl–
Cl–
Cl– Na+
Na
Na+ Cl–
+
Na+
Cl–
Cl–
Na+
Na+
Cl–
Na+ Cl–
Na+
Cl–
Cl–
Na+ Cl–
Na+ Cl–
Na+
44
45. 6.5
Structure and Physical Properties of
Ionic Compounds
Solubility of Ionic Compounds
• Usually soluble in water
Cl–
Na+ Cl– Na+
Cl
–
Na
+
Cl
dissolve in water
Na+
–
Na+ Cl– Na+
Na+
Water
molecules
•
URL
Cl–
Usually insoluble in organic solvents
E.g. ethanol, turpentine, petrol
45
46. 6.5
Structure and Physical Properties of
Ionic Compounds
Electrical Conductivity of Ionic Compounds
solid NaCl
aqueous NaCl
molten NaCl
46
47. 6.5
Structure and Physical Properties of
Ionic Compounds
Electrical Conductivity of Ionic Compounds
•
Ionic compounds conduct electricity in the molten
and aqueous states.
•
They do not conduct electricity in the solid state.
•
In the molten and aqueous states, mobile ions are
present.
•
Mobile ions conduct electricity.
47
Provide a brief introduction to the idea of noble gases.
Have students recall from Chapter 4 that neon, which is a noble gas, is monoatomic.
Have students recognise that noble gases exist in the monoatomic state.
State that noble gases are inert, i.e. their atoms do not combine with other atoms. Have students understand that the 8 electrons in the outer shell (2 in the case of helium) makes the atoms stable and, therefore, inert. A duplet or octet configuration is also known as a noble gas configuration.
Ask students why atoms of other elements react with one another. Lead them to understand that atoms of other elements react in order to achieve the stable noble gas structure.
Briefly introduce the two types of bonding.
Define Ions as charged particles formed when atoms lose or gain electrons.
Some students may have the misconception that when an ion gains an electron, it becomes positive and vice versa. Highlight to them that electrons are negatively charged, so when an atom gains an electron, a negative ion is formed.
Clicking on the URL button will link you to <http://www.bbc.co.uk/schools/gcsebitesize/science/add_aqa/atomic/ionicact.shtml>, a website with an interactive animation introducing the formation of ions and ionic bonds. (The video is approximately 3.5 minutes long.)
Mention that cations have a stable octet configuration in their outer shells.
Ask students to identify the noble gas configuration achieved by the sodium ion in its outer shell.
Ask students if they can identify the noble gas configuration of each ion, i.e. sodium ion has neon’s electronic configuration, calcium ion has argon’s electronic configuration.
Highlight to students that although the ions have the same electronic configuration as noble gases, they are not the same. For example, although sodium ion has the same configuration (2, 8) as neon, they are not identical particles because sodium ion has 11 protons, whereas neon has 10 protons.
Mention that anions have a stable octet configuration in their outer shells.
Ask students to identify the noble gas electronic configuration achieved by the chloride ion in its outer shell.
Emphasise that chlorine forms a chloride ion, not a chlorine ion.
Ask students if they can identify the noble gas configuration of each ion, e.g. chloride ion has argon’s electronic configuration.
Ask students:
What is another name for positive ions?
What is another name for negative ions?
Get students to recall that a metallic atom loses electron(s) to form a positive ion while a non-metallic atom gains electron(s) to form a negative ion. The oppositely charged ions are then held together by strong electrostatic forces of attraction.
Recap: Why do atoms form ions?
Answer: To achieve stable noble gas electronic configuration
Get students to identify the metal and non-metal elements in sodium chloride. Sodium is the metal, chlorine is the non-metal.
Clicking on the URL button will link you to <http://www.youtube.com/watch?v=QqjcCvzWwww&NR=1>, a website with a video on chemical bonding. The first part of the video (up till 0.51 seconds) can be used to demonstrate ionic bonding. The second part of the video (0.52 seconds onwards) may be used to demonstrate covalent bonding.
Alternative resource:
Go to <http://www.bbc.co.uk/schools/gcsebitesize/science/add_aqa/atomic/ionicrev4.shtml>, a website with an animation on ionic bonding of sodium chloride, magnesium oxide and calcium chloride.
Explain that although ions are charged particles, the compounds they form are neutral. Thus, we can infer that the positive and negative charges must be balanced.
When writing chemical formulae of ionic compounds, the metal is usually written first.
When there is only 1 of each ion, the subscript ‘1’ is not written. E.g. MgO and not Mg1O1
Highlight to students that polyatomic ions need to be enclosed within brackets when there is a subscript (in the case where there is more than one polyatomic ion).
Usually, the simplest set of whole numbers is written, e.g. CaCO3 and not Ca2(CO3)2. However, there are exceptions, e.g. H2O2 and N2O4.
Clicking on the URL button will link you to <http://www.learner.org/interactives/periodic/groups_interactive.html>, a website with an interactive game on ionic bonding. Students will get to match different ions to form compounds and see how well they score. (Note: There is an error in the description of aluminium phosphate – its chemical formula should be AlPO4, not AlPO2.)
The giant ionic structure is held together very tightly due to the strong electrostatic forces between the oppositely charged ions.
Another representation of the 3-D arrangement of Na+ and Cl− ions
Ionic compounds have a giant lattice structure, which is held very tightly by strong attractive forces. A large amount of energy is required to overcome these forces to change an ionic compound from the solid state to the liquid state. Therefore, ionic compounds have high melting and boiling points and are solids at r.t.p.
Clicking on the URL button will link you to <http://www.youtube.com/watch?v=EBfGcTAJF4o&feature=related>, a website with a video simulation on how water molecules “pull” the positive and negative ions in an ionic compound away from each other.
Sodium chloride does not conduct electricity in the solid state, but it can conduct in the molten and aqueous states.
A salt in the solid state, as shown earlier, has its ions held rigidly in fixed positions. It does not conduct electricity simply because the ions cannot move around. However, if we take the salt and dissolve it in water, the water molecules will pull the positive and negative ions away from each other. As a result, the ions will be free to move around to conduct electricity. Similarly, in the molten state, the ions will overcome some of the attractive forces between them and be free to move around. The presence of mobile ions enables electricity to be conducted.