7.1
Octet Rule
Learn the guiding principle behind the formation of many simple molecules.
Chemical Bonding
Understand why atoms combine, how the octet rule helps explain simple bonding, how Lewis structures represent shared electrons, and how the electron-sea model explains the characteristic properties of metals.
7.1.1
Lewis Approach
Represent valence electrons using Lewis symbols and show bonding electron pairs.
7.1.2
Exceptions to Octet Rule
Understand incomplete octets, expanded octets and odd-electron molecules.
7.2
Metallic Bonding
Study the electron-sea model and the collective nature of metallic bonding.
7.2.1
Electron Sea Model
See how mobile electrons and positive metal ions form a stable metallic structure.
7.2.2
Properties of Metals
Connect conductivity, malleability and ductility with mobile electrons.
Learning Sequence
1
Start with valence electrons.
See why atoms tend to achieve stable outer-shell arrangements.
See why atoms tend to achieve stable outer-shell arrangements.
2
Draw Lewis structures.
Use dots and lines to represent bonding and non-bonding electrons.
Use dots and lines to represent bonding and non-bonding electrons.
3
Recognise exceptions.
Not every stable molecule has exactly eight electrons around its central atom.
Not every stable molecule has exactly eight electrons around its central atom.
4
Move from molecules to metals.
See how delocalised electrons create metallic bonding and explain metal properties.
See how delocalised electrons create metallic bonding and explain metal properties.
7.1 Octet Rule
Atoms often become more stable when their valence shell reaches a noble-gas-like arrangement. For many atoms, this means having eight electrons in the outermost shell.
Octet rule: atoms other than hydrogen tend to form bonds by
gaining, losing or sharing electrons until they are surrounded by eight valence electrons.
Figure 7.1: Lewis covalent electron sharing in HF: Hydrogen achieves stable 2-electron duplet while Fluorine completes stable 8-electron octet.
Three ways atoms can reach a stable arrangement: gain electrons,
lose electrons, or share electrons with other atoms.
Quick Check
- What is meant by the octet rule?
- Why does hydrogen follow a duplet rather than an octet?
- Why should the octet rule be treated as a guiding principle rather than a law?
7.1.1 Lewis Approach
The Lewis approach represents the valence electrons of an atom as dots placed around its chemical symbol. These are called Lewis symbols.
Lewis symbol: the symbol of an element surrounded by dots
representing its valence electrons.
Example: Fluorine
Fluorine has atomic number 9 and electronic configuration \(2,7\). Therefore, it has seven valence electrons.
Lewis formula and bonding
During bonding, valence electrons are arranged so that the atoms move towards a stable outer-shell arrangement. In a covalent bond, electrons are shared between the combining atoms.
Duplet in HF: hydrogen has two electrons around it after sharing
the pair. This completes its first shell.
Quick Check
- What does a Lewis symbol represent?
- What do the dots between two atoms represent?
- What are non-bonding electrons?
- Why does hydrogen have a duplet in HF?
7.1.2 Exceptions to the Octet Rule
Many simple molecules follow the octet rule, but stable molecules can also exist with fewer than eight, more than eight, or an odd number of valence electrons around the relevant atom.
EXCEPTION 1
Incomplete Octet
The central atom has fewer than eight electrons. Examples discussed here include compounds of lithium, beryllium and boron; BF₃ is a common example.
EXCEPTION 2
Expanded Octet
The central atom has more than eight valence electrons. SF₆ has twelve electrons around sulfur.
EXCEPTION 3
Odd Number of Electrons
Molecules such as NO contain an odd number of valence electrons, leaving at least one unpaired electron.
Incomplete octet: BF₃
Boron has three valence electrons. In boron trifluoride, boron forms three bonds with three fluorine atoms. Six electrons surround the boron atom, so its octet remains incomplete.
Expanded octet: SF₆
Sulfur combines with six fluorine atoms in sulfur hexafluoride. Six S–F bonds place twelve electrons around the central sulfur atom.
Odd-electron molecule: NO
Nitrogen contributes five valence electrons and oxygen contributes six, giving a total of eleven valence electrons. Because the total is odd, at least one electron remains unpaired and at least one atom has an incomplete octet.
Quick Check
- Give one example of an incomplete-octet molecule.
- How many electrons surround boron in BF₃?
- How many electrons surround sulfur in SF₆?
- Why does NO have an unpaired electron?
- What happens whenever a molecule has an odd number of valence electrons?
7.2 Metallic Bonding
Metals such as iron, copper and aluminium can conduct electricity and heat, can be beaten into sheets, and can be drawn into wires. The electron-sea model provides a simple explanation for these properties.
Metallic bonding: the force of attraction between positive
metal ions and a sea of delocalised electrons.
Unlike a covalent bond, metallic bonding is not localised between a particular pair of atoms. The outer electrons are shared collectively by the metal structure.
Electron sea: outer electrons are not tied to one particular
atom. They can move throughout the metal while the positive metal ions remain arranged in a regular
structure.
Quick Check
- What is meant by the “electron sea”?
- Which particles remain in fixed positions in the model?
- Define metallic bonding.
- Why are metallic bonds non-directional?
7.2.1 Electron Sea Model and Properties of Metals
Electrical conductivity
When an electric field is applied to a metal, its mobile electrons can move in a particular direction. This movement of charge produces electric current. Therefore, metals are good conductors of electricity.
Thermal conductivity
When one part of a metal is heated, electrons in that region gain energy and move faster. They transfer energy to other parts of the metal. Vibrating metal ions also help transmit heat. This allows heat to spread quickly.
Malleability
Malleability is the ability of a metal to be beaten into thin sheets. The positive metal ions can slide over one another while the mobile electrons continue to hold the structure together.
Ductility
Ductility is the ability of a metal to be drawn into wires. As the metal is stretched, layers of ions can shift without breaking the non-directional metallic bonding.
Hardness and strength
Metals such as iron, copper and aluminium are commonly encountered as hard, useful engineering materials. The electron-sea model gives a simple picture of the collective bonding that holds the metal structure together. A detailed explanation of hardness and strength requires more advanced models of metallic bonding and the structure of solids.
| Property | Explanation using electron-sea model |
|---|---|
| Electrical conductivity | Mobile electrons move through the metal under an electric field. |
| Thermal conductivity | Mobile electrons transfer energy rapidly; vibrating ions also contribute. |
| Malleability | Ion layers can shift while the mobile electrons maintain attraction. |
| Ductility | Ion layers can slide during stretching without destroying the non-directional bonding. |
Quick Check
- How does the electron sea explain electrical conductivity?
- How does it explain thermal conductivity?
- What is malleability?
- What is ductility?
- Why do metals change shape instead of breaking when hammered?
- How is metallic bonding different from covalent bonding?
Metallic Bonding: A Simple Model
The electron-sea model is intentionally simple. It explains several important properties of metals without treating the outer electrons as completely lost from the metal. Instead, the electrons are shared collectively by the atoms in the solid.
Remember: metallic bonding is collective and non-directional.
This is why the bonding can continue even when layers of metal ions shift.
Exercise 7.1 — Chemical Bonding
Attempt each question first. Click Show Solution when you are ready to check your answer.
1. What is meant by the octet rule?
The octet rule states that atoms other than hydrogen tend to form bonds by
gaining, losing or sharing electrons until they are surrounded by eight valence electrons. It is
a guiding principle rather than a universal law.
2. Why does hydrogen not follow the octet rule?
Hydrogen has only its first shell, which is complete with two electrons.
Therefore, hydrogen follows a duplet configuration rather than an octet.
3. Give one example each of a molecule with (a) incomplete
octet, (b) expanded octet and (c) an odd electron.
(a) BF₃ — incomplete octet; (b) SF₆ — expanded octet; (c) NO — odd number of
valence electrons.
4. Why can boron form compounds with only six electrons around
it?
Boron has three valence electrons and can form three bonds. In BF₃, these
three bonds place six electrons around boron. Such an incomplete octet can still correspond to a
stable molecule.
5. What is meant by a duplet configuration?
A duplet configuration means that the first shell contains two electrons and
is complete. Hydrogen achieves this arrangement when it shares an electron pair in a bond.
6. Why is NO considered an exception to the octet rule?
Nitrogen contributes five valence electrons and oxygen contributes six, giving
eleven valence electrons in total. Since the total is odd, at least one electron remains
unpaired and at least one atom has an incomplete octet.
7. Draw the Lewis dot structure of BF₃ and explain why boron
does not complete its octet.
Boron is the central atom and forms three B–F bonds. Each fluorine completes
its own octet, but only six electrons surround boron. Hence boron has an incomplete octet.
8. Assertion: SF₆ violates the octet rule. Reason: Sulphur can
accommodate more than eight electrons. Choose the correct option.
Answer: A. Both the assertion and reason are correct, and the
reason explains why SF₆ has an expanded octet.
9. What is meant by the term “electron sea” in metals?
It is the collection of delocalised outer electrons that can move throughout a
metal rather than remaining attached to one particular atom.
10. What type of particles are in fixed positions in a metal
according to the electron-sea model?
Positive metal ions are arranged in a regular structure. The mobile electrons
move around and between these ions.
11. Define metallic bonding.
Metallic bonding is the force of attraction between positive metal ions and
the sea of delocalised electrons.
12. Why are metallic bonds called non-directional?
The electrons are shared collectively throughout the metal rather than being
localised between a particular pair of atoms. Therefore the attraction is not restricted to one
direction between two atoms.
13. Name two properties of metals explained by the electron-sea
model.
Electrical conductivity and thermal conductivity are two properties explained
by the model. It also helps explain malleability and ductility.
14. Explain how the electron-sea model accounts for electrical
conductivity in metals.
Metals contain mobile electrons. When an electric field is applied, these
electrons move in a particular direction and produce electric current.
15. How does the electron-sea model explain thermal
conductivity?
Electrons near the heated region gain energy and transfer it as they move
through the metal. Vibrating metal ions also help transfer heat, so heat spreads quickly.
16. Why can metals be beaten into thin sheets?
The layers of positive metal ions can slide over one another while the mobile
electrons continue to maintain attraction between the ions. Therefore the metal changes shape
instead of breaking.
17. What is meant by ductility? How is it explained by the
electron-sea model?
Ductility is the ability of a metal to be drawn into wires. During stretching,
metal ions can shift relative to one another while the non-directional metallic bonding remains
effective.
18. How is metallic bonding different from covalent bonding?
In covalent bonding, electrons are shared between specific atoms. In metallic
bonding, the outer electrons are delocalised and shared collectively by the metal atoms, making
the bonding non-directional.
19. Explain the structure of a metal according to the
electron-sea model.
A metal consists of positive metal ions arranged in a regular structure and a
sea of mobile, delocalised electrons moving around and between them. The attraction between the
ions and electrons holds the metal together.
20. If electrons in a metal were not free to move, which
property would be most affected? Explain.
Electrical conductivity would be most directly affected because electric
current in a metal depends on the movement of its mobile electrons.
21. Explain why metals do not break when hammered but instead
change shape.
The layers of metal ions can shift while the delocalised electrons continue to
provide attraction between them. Since metallic bonding is non-directional, the structure can
adjust without the bonding being completely broken.
22. Copper is used for electrical wiring, while rubber is not.
Explain using the electron-sea model.
Copper is a metal and contains mobile electrons that can move under an
electric field, allowing electric current to pass. Rubber does not have the same sea of mobile
electrons and therefore does not conduct electricity in the same way.
23. Why are metals generally good conductors of heat compared
with nonmetals?
Mobile electrons in metals can transfer energy rapidly through the material.
Vibrations of metal ions also contribute to heat transfer.
24. Assertion: Metals are good conductors of electricity.
Reason: Metals contain free electrons that can move under an electric field. Choose the correct
option.
Answer: A. Both the assertion and reason are correct, and the
reason correctly explains the assertion.
25. Assertion: Metallic bonds are non-directional. Reason:
Electrons in metals are localised between two atoms. Choose the correct option.
Answer: C. The assertion is correct, but the reason is wrong.
Electrons in metallic bonding are delocalised rather than localised between two atoms.
26. Assertion: Metals are malleable. Reason: Layers of metal
ions can slide while electrons continue to hold them together. Choose the correct option.
Answer: A. Both the assertion and reason are correct, and the
reason correctly explains the assertion.
Worksheets
Practice worksheets for Lewis structures, octet-rule exceptions and metallic bonding will be added here.
Coming SoonQuick Revision
| Topic | Key point |
|---|---|
| Octet rule | Atoms often gain, lose or share electrons to achieve eight valence electrons. |
| Duplet | Hydrogen achieves a stable first shell with two electrons. |
| Lewis symbol | Dots around an element symbol represent its valence electrons. |
| Bonding pair | A shared electron pair between two bonded atoms. |
| Non-bonding electrons | Valence electrons that do not participate in the bond. |
| Incomplete octet | Stable molecules can have fewer than eight electrons around a central atom; BF₃ is an example. |
| Expanded octet | Stable molecules can have more than eight electrons around a central atom; SF₆ has twelve around sulfur. |
| Odd-electron molecule | An odd total number of valence electrons leaves at least one unpaired electron; NO is an example. |
| Electron sea | Delocalised outer electrons move throughout a metal. |
| Metallic bonding | Attraction between positive metal ions and delocalised electrons. |
| Electrical conductivity | Mobile electrons move under an electric field. |
| Thermal conductivity | Mobile electrons and vibrating ions transfer energy. |
| Malleability | Metal can be beaten into sheets because ion layers can shift while bonding remains. |
| Ductility | Metal can be drawn into wires because ion layers can slide during stretching. |
Test Yourself
A chapter test covering the octet rule, Lewis approach, exceptions and metallic bonding will be added here.
Coming Soon