Q.Explain the non linear shape of H2S and non planar shape of PCl3 using valence shell electron pair repulsion theory.

The Valence Shell Electron Pair Repulsion (VSEPR) theory states that electron pairs around a central atom arrange themselves to minimize repulsion, determining the molecule's geometry. Lone pair-lone pair repulsions are stronger than lone pair-bond pair repulsions, which are stronger than bond pair-bond pair repulsions.

For H$_2$S:

1. The central atom is sulfur (S), which has 6 valence electrons. It forms single bonds with two hydrogen (H) atoms, using 2 electrons.
2. The remaining $6 - 2 = 4$ electrons form two lone pairs on the sulfur atom.
3. Thus, sulfur has two bond pairs and two lone pairs, leading to a steric number of 4. The electron geometry is tetrahedral.
4. Due to the stronger repulsions from the two lone pairs, the bond angle is compressed from the ideal $109.5^\circ$, resulting in a bent or V-shaped molecular geometry, which is non-linear.

For PCl$_3$:

1. The central atom is phosphorus (P), which has 5 valence electrons. It forms single bonds with three chlorine (Cl) atoms, using 3 electrons.
2. The remaining $5 - 3 = 2$ electrons form one lone pair on the phosphorus atom.
3. Thus, phosphorus has three bond pairs and one lone pair, leading to a steric number of 4. The electron geometry is tetrahedral. …

Valence Shell Electron Pair Repulsion (VSEPR) theory is a model used to predict the three-dimensional geometry of molecules. Its fundamental principle is that electron pairs, being negatively charged, repel each other and will arrange themselves in space to minimize this repulsion, thus maximizing the distance between them. This arrangement defines the electron geometry around the central atom.

However, not all electron pairs are equivalent in their repulsive strength. Lone pairs (non-bonding electrons) occupy more space than bonding pairs (electrons shared between two atoms) because lone pairs are attracted to only one nucleus, allowing them to spread out more. Bonding pairs are attracted to two nuclei, which constrains their spatial distribution. This difference leads to a specific order of repulsion:

Lone Pair - Lone Pair (LP-LP) > Lone Pair - Bonding Pair (LP-BP) > Bonding Pair - Bonding Pair (BP-BP)

This differential repulsion causes distortions from ideal geometries, leading to the observed molecular shapes and affecting bond angles.

Let's apply VSEPR theory to understand the shapes of $\text{H}_2\text{S}$ and $\text{PCl}_3$.

Shape of $\text{H}_2\text{S}$

1. Identify the Central Atom and Valence Electrons:

   Sulfur (S) is the central atom in $\text{H}_2\text{S}$. Sulfur is in Group 16, contributing 6 valence electrons. Each hydrogen (H) atom is in Group 1, contributing 1 valence electron.

   Total valence electrons = $6 \text{ (from S)} + 2 \times 1 \text{ (from 2 H)} = 8$ valence electrons.

2. Draw the Lewis Structure:

   The Lewis structure shows sulfur bonded to two hydrogen atoms. Each S-H bond uses 2 electrons, totaling 4 electrons for bonding. The remaining $8 - 4 = 4$ electrons form two lone pairs on the central sulfur atom.

   $\text{H} - \ddot{\text{S}} - \text{H}$

3. Count Electron Domains:

   Around the central sulfur atom, there are:

   • 2 bonding pairs (S-H bonds)
   • 2 lone pairs Total electron domains = $2 \text{ (bonding pairs)} + 2 \text{ (lone pairs)} = 4$ electron domains.

4. Determine Electron Geometry:

   According to VSEPR theory, 4 electron domains will arrange themselves in a tetrahedral electron geometry to minimize repulsion.

5. Determine Molecular Geometry:

   The molecular geometry describes the arrangement of only the atoms. The two lone pairs on the sulfur atom exert stronger LP-LP and LP-BP repulsions than the BP-BP repulsion between the S-H bonds. This increased repulsion pushes the two S-H bonds closer together, distorting the ideal tetrahedral arrangement.

   The resulting molecular geometry is bent or V-shaped. This is a non-linear shape.

6. Bond Angle:

   In an ideal tetrahedral geometry (e.g., $\text{CH}_4$), the bond angle is $109.5^\circ$. Due to the stronger LP-BP repulsion in $\text{H}_2\text{S}$, the $\text{H}-\text{S}-\text{H}$ bond angle is compressed to approximately $92^\circ$.

Shape of $\text{PCl}_3$

1. Identify the Central Atom and Valence Electrons:

   Phosphorus (P) is the central atom in $\text{PCl}_3$. Phosphorus is in Group 15, contributing 5 valence electrons. Each chlorine (Cl) atom is in Group 17, contributing 7 valence electrons.

   Total valence electrons = $5 \text{ (from P)} + 3 \times 7 \text{ (from 3 Cl)} = 5 + 21 = 26$ valence electrons.

2. Draw the Lewis Structure:

   The Lewis structure shows phosphorus bonded to three chlorine atoms. Each P-Cl bond uses 2 electrons, totaling 6 electrons for bonding. The remaining $26 - 6 = 20$ electrons are distributed as lone pairs. Each chlorine atom needs 6 electrons (3 lone pairs) to complete its octet, accounting for $3 \times 6 = 18$ electrons. The remaining $20 - 18 = 2$ electrons form one lone pair on the central phosphorus atom.

   $\text{Cl} - \ddot{\text{P}} - \text{Cl}$

   $\quad\quad\quad |$

   $\quad\quad\quad\text{Cl}$

3. Count Electron Domains:

   Around the central phosphorus atom, there are:

   • 3 bonding pairs (P-Cl bonds)
   • 1 lone pair Total electron domains = $3 \text{ (bonding pairs)} + 1 \text{ (lone pair)} = 4$ electron domains.

4. Determine Electron Geometry:

   Similar to $\text{H}_2\text{S}$, 4 electron domains will arrange themselves in a tetrahedral electron geometry to minimize repulsion.

5. Determine Molecular Geometry: …