Problem 18 - A Complete Resource Book in Chemistry for JEE Main

Question

Which of the following species has a linear shape? (a) \(\mathrm{NO}_{2}^{+}\) (b) \(\mathrm{O}_{3}\) (c) \(\mathrm{NO}_{2}^{-}\) (d) \(\mathrm{SO}_{2}\)

Step-by-Step Solution

Answer

The species \\(\mathrm{NO}_{2}^{+}\\) has a linear shape.

Step 1: Understanding Linear Shape

Molecules with a linear shape have bond angles of 180 degrees. This typically occurs in diatomic molecules or when the central atom forms only two bonds without any lone pairs, or if lone pairs do not affect the molecular geometry.

Step 2: Analyze \\(\mathrm{NO}_{2}^{+}\\)

The \(\mathrm{NO}_{2}^{+}\) ion has a central nitrogen atom bonded to two oxygen atoms. With no lone pairs on the nitrogen and a total of two electron regions around it, the molecular shape is linear.

Step 3: Analyze \\(\mathrm{O}_{3}\\)

The \(\mathrm{O}_{3}\) (ozone) molecule consists of three oxygen atoms with the central oxygen atom having one lone pair, leading to a bent shape due to the lone pair repulsion.

Step 4: Analyze \\(\mathrm{NO}_{2}^{-}\\)

In the \(\mathrm{NO}_{2}^{-}\) ion, the nitrogen is surrounded by two oxygen atoms and has one lone pair, which gives the molecule a bent shape similar to that of a water molecule.

Step 5: Analyze \\(\mathrm{SO}_{2}\\)

The \(\mathrm{SO}_{2}\) molecule has a central sulfur atom with two oxygen atoms bonded to it and one lone pair. This lone pair results in a bent molecular shape.

Key Concepts

Understanding Linear Shape in Molecules

Linear molecular geometry is a fascinating concept in chemistry where the molecule takes on a straight-line shape. This typically happens when there are two atoms bonded to a central atom, with a bond angle of 180 degrees. Imagine this like a straight stick where both ends are equally important in maintaining the symmetry.
Linear shapes are common in diatomic molecules like carbon dioxide

• Molecules with linear shapes have bond angles of 180 degrees.
• Occurs in diatomic molecules or when a central atom forms two bonds.

The absence of lone pairs significantly contributes to this geometry by allowing bonded atoms to spread equally apart, resulting thus in a linear arrangement.

Valence Shell Electron Pair Repulsion Theory (VSEPR Theory)

VSEPR Theory, or Valence Shell Electron Pair Repulsion Theory, is vital to understanding molecular shapes. This theory proposes that electron pairs around a central atom will arrange themselves as far apart as possible to minimize repulsion. Think of electrons like guests at a party who try to avoid crowding each other. They naturally spread out to enjoy the maximum space without being disturbed by another guest.
The beauty of this theory is in its simplicity and ability to predict molecular structures easily based on electron pair arrangements.

• Electron pairs spread to minimize repulsion.
• Helps predict the geometric arrangement of compounds.
• Accounts for both bonding and non-bonding pairs, altering the geometry.

The Role of Lone Pairs in Molecular Geometry

Lone pairs play an intriguing part in shaping molecules. They are the valence electrons not involved in bonding. Though they don't form a 'rope' connecting atoms, their presence is weighty in shaping the molecular structures.
Lone pairs are like invisible players that push bonded atoms away, affecting the overall angle between bonds. This sometimes tweaks the molecule's shape unexpectedly.

• Lone pairs are non-bonding pairs of electrons.
• They affect molecular shape by exerting repulsion on bonded pairs.
• Presence of lone pairs tends to decrease bond angles.

Decoding Bond Angles

Bond angles are another crucial piece of the molecular shape puzzle. They measure the angle between two adjacent bonds at an atom. Bond angles tell us how space is shared within a molecule and help determine its overall shape.
In a linear shape, the bond angle is 180 degrees, which is the angle formed when atoms are as far apart as they can possibly be in a straight line. When lone pairs come into play, they create a repulsion zone that decreases bond angles slightly, causing the molecule to adjust its shape to accommodate.

• Bond angles refer to angles between two bonds emanating from a central atom.
• Useful in predicting and explaining geometric shapes of molecules.
• Impact of lone pairs can modify these angles, making them smaller than expected.