The shape of an ammonia molecule is

KPSC Staff Nurse - 2015

Biochemistry and Nutrition

Medium

Appeared in: KPSC Staff Nurse - 2015

Explanation

The central nitrogen atom in ammonia (NH₃) has 5 valence electrons.
It forms three single covalent bonds with hydrogen atoms and has one non-bonding lone pair of electrons.
The four electron domains (3 bonding pairs + 1 lone pair) create a tetrahedral electron geometry.
Due to the presence of the lone pair, which is not considered in the final shape, the molecular geometry is trigonal pyramidal.
The strong repulsion from the lone pair compresses the H-N-H bond angles from the ideal 109.5° to approximately 107°.

Option A: This describes the arrangement of the four electron pairs (3 bonding, 1 lone) around the central nitrogen atom, which is indeed tetrahedral. However, the molecular shape only considers the positions of the atoms, which are arranged in a pyramid due to the influence of the unseen lone pair.
Option B: A linear shape requires only two electron domains around the central atom, resulting in a 180° bond angle. Ammonia has four electron domains.
Option D: A planar shape, specifically trigonal planar, involves three electron domains and no lone pairs, with 120° bond angles. Ammonia has four electron domains, including a lone pair.

Clinical Relevance

Nursing practice connection: This is primarily an exam-oriented knowledge point with limited direct bedside application, so retain Molecular geometry and VSEPR theory as background academic context rather than a clinical decision trigger.
Understanding molecular shapes is a fundamental concept in biochemistry. It helps predict how molecules, such as amino acids (which have an -NH₂ group), neurotransmitters, and drugs, will interact with specific biological receptors in the body.
The pyramidal shape of the amino group (-NH₂) and its lone pair are crucial for forming hydrogen bonds, which are essential for the secondary and tertiary structures of proteins, as seen in alpha-helices and beta-sheets.
The shape of a molecule determines its polarity. The pyramidal shape and the lone pair make ammonia a polar molecule, allowing it to be highly soluble in water. This is critical in physiology, as ammonia is a toxic byproduct of metabolism that must be transported in the blood (an aqueous medium) to the liver for detoxification.

How to Approach the Question

First, identify the central atom of the molecule, which is Nitrogen (N) in ammonia (NH₃).
Determine the number of valence electrons for the central atom. Nitrogen is in Group 15, so it has 5 valence electrons.
Count the number of atoms bonded to the central atom. In NH₃, there are 3 Hydrogen atoms bonded to Nitrogen.
Calculate the number of lone pairs on the central atom: [Total Valence e⁻ - (Number of bonds)] / 2. For NH₃, this is [5 - 3] / 2 = 1 lone pair.
Determine the total number of electron domains around the central atom by adding the bonding pairs and lone pairs. Here, it is 3 + 1 = 4 domains.
Based on VSEPR theory, 4 electron domains result in a tetrahedral electron geometry.

Concept Tested & Keywords

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