Chemical Bonding Class 9 Notes Advanced Science Chapter 7

Reviewing Class 9 Advanced Science Notes and Chapter 7 Chemical Bonding Class 9 Notes regularly helps in retaining important facts.

Chemical Bonding Notes Class 9 Advanced Science

Class 9 Chemical Bonding Notes

Octet Rule

Atoms with eight electrons in their valence or outermost shell are stable. Most atoms (other than hydrogen) achieve this by gaining, losing or sharing electrons during bond formation and acquire (or achieved) eight valence electrons. This is called the octet rule.

→ Key points about the octet rule:

• It is a useful guiding principle for understanding simple molecule formation not a law.
• Hydrogen is an exception as its valence shell follows the duplet rule, attaining the configuration of helium (two electrons).
• Many stable molecules do not follow the octet rule, these are known as exceptions.

→ Lewis Approach:

• Lewis symbol: The valence electrons of an atom are represented as dots placed around the atomic symbol. Each dot represents one valence electron.
• For example, Fluorine (atomic number = 9) has the electronic configuration 2, 7 giving it 7 valence electrons. Its Lewis symbol is

Bonding using Lewis symbols During bond formation, the valence electrons of each atom are written around their symbols. Electrons are then shared so that each atom completes its octet.

→ For example, in hydrogen fluoride (HF):

• Hydrogen contributes 1 valence electron, fluorine contributes 7.
• The pair of dots between the atoms represents the bonding electrons (shared pair).
• The remaining dots around fluorine are non-bonding electrons (lone pairs), they do not participate in bonding.
• Hydrogen ends up with only 2 electrons (a duplet), which is its stable configuration.

→ Exceptions to the Octet Rule:

Many stable molecules exist that do not obey the octet rule. There are three main types of exceptions

(a) Molecules with Incomplete Octets

Some elements have fewer than four valence electrons in their outermost shell, so they cannot form enough bonds to complete an octet. They also lack sufficient lone pairs for this purpose.

Example 1:
Lithium (1 valence e^–), beryllium (2 valence e^–) and boron (3 valence e^–) can form only 1, 2, and 3 bonds respectively.
Answer:
Their central atoms end up with only 2, 4, and 6 electrons below 8 valence electrons.
Despite this, such molecules are stable. A common example is boron trifluoride (BF₃), where boron forms 3 bonds and has only 6 electrons around it.

(b) Molecules with Expanded Octets
Some atoms can accommodate more than eight electrons in their valence shell called an expanded octet. This occurs in elements that have more than four valence electrons.

Example 2.
In sulphur hexafluoride (SF₆), one sulphur atom bonds with six fluorine atoms.
Answer:
The central sulphur atom ends up with 12 electrons around it, an expanded octet.
Such compounds are formed by elements in the third period and beyond (e.g. S, P, Cl) which have access to d-orbitals.

(Formation of such compounds will be studied in detail in higher classes.)

(c) Molecules with Odd Number of Electrons
Some molecules have an odd number of valence electrons. In such cases, it is impossible for every atom to achieve a complete octet.

Example 3.
Nitric oxide (NO), nitrogen has 5 valence electrons, oxygen has 6, giving a total of 11 (odd number).
Answer:
Since 11 is odd, at least one atom will always have an incomplete octet.
There will always be one unpaired electron in such molecules.

(The two lines in the bond line structure indicate a double bond between N and O.)

Metallic Bonding

• Metals like iron, copper and aluminium share common properties, they are hard, malleable, ductile and good conductors of electricity and heat. These properties can be explained using a simple model called the electron sea model.
• Unlike ionic or covalent bonding, where electrons are transferred or shared between specific atoms, the metallic bonding involves a large number of atoms collectively. The electron sea model describes this collective interaction.

→ Electron Sea Model:

In a metal, the outermost electrons of each atom are loosely held by the nucleus. When many metal atoms come together to form a solid, these outer electrons become delocalised, they are no longer attached to any single atom and are free to move throughout the entire piece of metal.

→ The structure of a metal according to this model

• Metal atoms lose their outer electrons and become positive metal ions (cations).
• These positive ions arrange themselves in a regular, fixed pattern, forming a lattice structure.
• The released electrons move freely and randomly in all directions around and between these ions forming a sea of electrons.
• The strong attraction between the positive metal ions and this sea of free electrons holds the metal together, this is called metallic bonding.

Metallic bonding is defined as the force of attraction between positive metal ions and the surrounding sea of delocalised electrons. Unlike covalent bonds, metallic bonds are

• Non-localised electrons are shared collectively by all atoms, not between specific pairs.
• Non-directional the bond has no fixed direction and can adjust to shifting ion positions.

Electron Sea Model and Properties of Metals

The electron sea model correctly explains the following key physical properties of metals

→ Electrical Conductivity:

• Free electrons in a metal can move throughout the structure.
• When an electric field is applied, these electrons move in a specific direction — this directed flow of electrons constitutes electric current.
• This is why metals are good conductors of electricity.

→ Thermal Conductivity:

• When one part of a metal is heated, the electrons in that region gain kinetic energy and move faster.
• As they move, they transfer this energy to neighbouring regions of the metal.
• Simultaneously, the metal ions vibrate more vigorously, also helping conduct heat. This is why metals are good conductors of heat.

→ Malleability:

Malleability is the ability of a metal to be beaten into thin sheets.

• When a force is applied, layers of positive metal ions slide over one another.
• The free electrons, being mobile — continue to hold the ions together even after shifting.
• Since the metallic bonds are non-directional, the structure does not break, the metal changes its shape.

→ Ductility:

Ductility is the ability of a metal to be drawn into wires.

• When a metal is stretched, the ions slide past each other without breaking the non-directional metallic bonds.
• The sea of electrons continuously adjusts to maintain the attraction between ions even as the shape changes.
• This allows metals to be stretched into long, thin wires.

Note:
The electron sea model gives a simplified picture of metallic bonding. Unlike ionic bonding, the electrons are not completely lost — they are shared collectively by all atoms. More detailed models will be studied in higher classes.

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