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Electron pair repulsion theory explains the specific shapes of the molecules of methane, boron trifluoride and ammonia and of molecules with general formula QX₂ - Leaving Cert Chemistry - Question 10 - 2021
Question 10
Electron pair repulsion theory explains the specific shapes of the molecules of methane, boron trifluoride and ammonia and of molecules with general formula QX₂. (i... show full transcript
Worked Solution & Example Answer:Electron pair repulsion theory explains the specific shapes of the molecules of methane, boron trifluoride and ammonia and of molecules with general formula QX₂ - Leaving Cert Chemistry - Question 10 - 2021
Step 1
Draw a dot and cross diagram to show the arrangement of the valence electrons in an NH₃ molecule.
Answer
In the NH₃ molecule, nitrogen (N) has five valence electrons and each hydrogen (H) has one. You can illustrate the dot and cross diagram by representing the three hydrogen atoms connected to the nitrogen atom. The nitrogen will have one lone pair of electrons. Therefore, the diagram will show three N-H bonds and one lone pair on nitrogen:
Step 2
Why are NH₃ molecules not trigonal planar like BF₃ molecules?
Answer
NH₃ is not trigonal planar due to the presence of a lone pair on the nitrogen atom, which influences its geometry. In NH₃, there are four electron pairs (three bond pairs and one lone pair) that are arranged in a tetrahedral geometry, but since lone pairs occupy more space, the actual shape becomes trigonal pyramidal, rather than trigonal planar as seen in BF₃, which has no lone pairs.
Step 3
What are the two possible shapes of a molecule with general formula QX₂?
Answer
The two possible shapes for a molecule with the general formula QX₂ are:
- Linear shape, if there are no lone pairs on the central atom Q.
- Bent or V-shaped, if there are one or more lone pairs on the central atom Q.
Step 4
Explain why both CH₄ and BF₃ are non-polar molecules.
Answer
Both CH₄ and BF₃ are non-polar due to their symmetrical molecular geometry. In CH₄, the tetrahedral arrangement of hydrogen atoms around the central carbon atom means that the dipole moments cancel out. Similarly, in BF₃, the trigonal planar shape allows for equal bond angles, resulting in a cancellation of dipole moments, making the molecule non-polar.
Step 5
What evidence is there in the table to support the claim that bond-pair bond-pair (bp-bp) repulsions are weaker than lone-pair bond-pair (lp-bp) repulsions?
Answer
The boiling point data supports this claim. In the table, NH₃ has a higher boiling point compared to CH₄, signifying stronger intermolecular forces due to higher lone-pair bond-pair repulsions compared to bond-pair bond-pair repulsions observed in CH₄. Additionally, the bond angles indicate that lone pairs exert more repulsion on the bond pairs, indicating that lp-bp interactions are stronger than bp-bp interactions.