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Use a dot and cross diagram to show the bonding in an ammonia, NH₃, molecule - Leaving Cert Chemistry - Question b - 2009
Question b
Use a dot and cross diagram to show the bonding in an ammonia, NH₃, molecule. Use electron pair repulsion theory to determine the shape of the ammonia molecule. Exp... show full transcript
Worked Solution & Example Answer:Use a dot and cross diagram to show the bonding in an ammonia, NH₃, molecule - Leaving Cert Chemistry - Question b - 2009
Step 1
Use a dot and cross diagram to show the bonding in an ammonia, NH₃, molecule.
Answer
To illustrate the bonding in ammonia (NH₃), we use a dot and cross diagram. Each nitrogen atom contributes three valence electrons, while each hydrogen atom contributes one. In the diagram, we represent the electrons from nitrogen as dots and the electrons from hydrogen as crosses.
In NH₃, the nitrogen forms three covalent bonds with three hydrogen atoms, leading to a structure that looks like this:
H
|
H - N ·
|
H
This shows the shared pairs of electrons between nitrogen and each hydrogen atom.
Step 2
Use electron pair repulsion theory to determine the shape of the ammonia molecule. Explain clearly why the bond angle in ammonia is only 107°.
Answer
According to the electron pair repulsion theory (VSEPR theory), the shape of the ammonia molecule is determined by the repulsion between the electron pairs surrounding the central nitrogen atom.
There are four pairs of electrons around nitrogen: three bonding pairs (with hydrogen) and one lone pair. The shape of the molecule is pyramid-shaped with the nitrogen atom at the apex and hydrogen atoms at the corners of the base. The lone pair takes up more space, causing the bonding pairs to repel each other slightly more than in a tetrahedral arrangement. This results in the bond angle being slightly less than the ideal tetrahedral angle of 109.5°, specifically around 107°.
Step 3
What are hydrogen bonds? Draw a diagram illustrating hydrogen bonding in ammonia.
Answer
Hydrogen bonds are a type of intermolecular attraction that occurs between molecules when a hydrogen atom bonded to a highly electronegative atom (like nitrogen) experiences an attraction to another electronegative atom in a neighboring molecule.
In the case of ammonia, hydrogen bonds form between the hydrogen atoms of one ammonia molecule and the lone pair of electrons on the nitrogen atom of another ammonia molecule. This interaction can be illustrated as follows:
H H
| |
H - N···H - N
| |
H H
In this illustration, the dotted line represents the hydrogen bond between the nitrogen of one molecule and the hydrogen of another, showing the intermolecular nature of hydrogen bonds.