Problem 151 - A Complete Resource Book in Chemistry for JEE Main

Question

Consider the following statements: (1) in diamond, each carbon atom is linked tetrahedrally to four other carbon atoms by \(\mathrm{sp}^{3}\) bonds. (2) graphite has planar hexagonal layers of carbon atoms held together by weak Van der Waal's forces. (3) silicon exists only in diamond structure due to its tendency to form p\pi-p\pi bonds to itself. (a) only 3 is correct (b) only 1 and 2 are correct (c) only 2 and 3 are correct (d) all are correct statements

Step-by-Step Solution

Answer

(b) only 1 and 2 are correct

Step 1: Analyze Statement 1

In diamond, each carbon atom is indeed linked tetrahedrally to four other carbon atoms using \( \mathrm{sp}^{3} \) hybridization. This structure is very strong and is responsible for diamond's hardness. Statement 1 is correct.

Step 2: Analyze Statement 2

Graphite consists of planar layers of carbon atoms arranged in hexagonal patterns. These layers are held together by weak Van der Waals forces, which allow them to slide over each other easily. Statement 2 is correct.

Step 3: Analyze Statement 3

Silicon forms a structure similar to diamond, which is based on \( \mathrm{sp}^{3} \) tetrahedral bonding. However, silicon does not form \( p\pi-p\pi \) bonds due to its larger atomic size compared to carbon, which hinders \( p\pi-p\pi \) overlap. Thus, Statement 3 is incorrect.

Step 4: Determine the Correct Answer

From the analysis, we found that Statements 1 and 2 are correct, while Statement 3 is not correct. Therefore, the answer to the question is that only Statements 1 and 2 are correct.

Key Concepts

Diamond Structure

In a diamond, each carbon atom is connected to four other carbon atoms in a three-dimensional network.
This structure is known as tetrahedral, as the connections form a shape similar to a pyramid with a triangular base, called a tetrahedron.
This arrangement is due to the carbon atoms forming \( sp^3 \) bonds.
As a result, the diamond structure provides great strength and hardness.
This is why diamonds are well-known for their impressive durability.

• The tetrahedral bonding angle is precisely 109.5°, optimizing the spatial arrangement.
• Every carbon atom in the network is equidistant to its neighbors, making diamonds extremely stable.

Due to this rigid formation, diamonds do not conduct electricity but are excellent heat conductors.
This structure is purely covalent, meaning that the atoms in a diamond share electrons to form strong bonds.

Graphite Structure

Graphite is well-known for its layered structure of carbon atoms, which are organized in a planar hexagonal lattice.
Each carbon atom in a single layer is bonded to three others through covalent \( sp^2 \) hybridization, leaving one free electron.
These free electrons contribute to the conductivity of graphite.

• This free electron allows electricity to be conducted along the layers.
• Graphite is much less hard than a diamond due to the bonding structure between layers.

These layers are stacked with weak Van der Waals forces acting between them.
Because these forces are weak, the layers can slide over one another effortlessly.
This sliding behavior gives graphite its properties as a good lubricant and its ability to produce marks on paper, which is why it is used in pencils.
The contrast between the strong covalent bonds within a layer and the weak forces between layers makes graphite unique among allotropes of carbon.

SP3 Hybridization

SP3 hybridization is a key concept for understanding why diamond has its particular structure.
When carbon atoms undergo \( sp^3 \) hybridization, one s orbital and three p orbitals within the atom mix to form four new hybrid orbitals.
These orbitals are identical in energy and spatial orientation, allowing carbon to form four strong sigma bonds.

• This mixing process results in a more energetically favorable state for the carbon atom.
• The sp3 orbitals are directed towards the corners of a tetrahedron.

With these strong bonds, each carbon atom connects with four others, creating a stable 3D structure.
Without \( sp^3 \) hybridization, the characteristic properties of diamond, such as its unrivaled hardness and high thermal conductivity, would not be possible.
This concept is not only important for diamond but also for other tetrahedral structures in chemistry, signifying a wide range of applicability.

Van der Waals Forces

Van der Waals forces are weak, attractive forces that occur between molecules.
These forces are responsible for holding together certain types of structures, such as the layers in graphite.

• They are the result of temporary fluctuations in electron distribution around molecules or atoms that lead to transient dipoles.
• Although weak, they can be summed over large areas to contribute significant stability to a structure's layers.

In the case of graphite, these forces permit the layers to maintain a close proximity without strong bonds, enabling them to slide.
While not as strong as ionic or covalent bonds, Van der Waals forces are essential for the mechanical properties of various materials.
They describe why materials like graphite are soft and able to act as excellent lubricants.
These forces allow for flexibility and resilience in the structure, enabling many potential uses in both scientific and industrial applications.