b. The NH3 molecule is trigonal pyramidal, while BF3 is trigonal planar. Which of these molecules is flat?

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Identify the molecular geometry of each molecule: NH3 is trigonal pyramidal, and BF3 is trigonal planar.

Understand the characteristics of each geometry: Trigonal pyramidal has a three-dimensional shape due to the presence of a lone pair on the central atom, while trigonal planar is flat with all atoms in the same plane.

Consider the effect of lone pairs: In NH3, the lone pair on nitrogen pushes the hydrogen atoms down, creating a three-dimensional shape.

Recognize that BF3 has no lone pairs on the central boron atom, allowing all atoms to lie in the same plane.

Conclude that the molecule with a trigonal planar geometry, BF3, is flat, while NH3 is not flat due to its trigonal pyramidal shape.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Molecular Geometry

Molecular geometry refers to the three-dimensional arrangement of atoms within a molecule. It is determined by the number of bonding pairs and lone pairs of electrons around the central atom, which influences the shape. For example, NH3 (ammonia) has a trigonal pyramidal shape due to one lone pair, while BF3 (boron trifluoride) has a trigonal planar shape with no lone pairs.

Trigonal Planar Geometry

Trigonal planar geometry occurs when a central atom is bonded to three other atoms, with all bonds lying in the same plane. This arrangement minimizes electron pair repulsion, resulting in bond angles of approximately 120 degrees. BF3 exemplifies this geometry, making it a flat molecule.

Trigonal Pyramidal Geometry

Trigonal pyramidal geometry is characterized by a central atom bonded to three other atoms and having one lone pair of electrons. This lone pair pushes down on the bonded atoms, creating a pyramid-like shape with bond angles less than 109.5 degrees. NH3 is an example of this geometry, which results in a non-flat, three-dimensional structure.

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