Question

Which of the following molecules has a trigonal planar shape?

• A. \( \mathrm{CH_4} \)
• B. \( \mathrm{BF_3} \)
• C. \( \mathrm{NH_3} \)
• D. \( \mathrm{H_2O} \)

Hint:

Tip: Use VSEPR theory to determine molecular shape based on bonding and lone pairs.

Answer 1

The Correct Option is B

To determine which of the given molecules has a trigonal planar shape, we first need to understand the concept of molecular geometry. The shape of a molecule is determined by the arrangement of atoms around the central atom, which is influenced by the electron-pair repulsion theory, often called the VSEPR (Valence Shell Electron Pair Repulsion) theory. 

VSEPR Theory: The shape of a molecule is determined by the repulsions between electron pairs in the valence shell of the central atom. These pairs arrange themselves as far apart as possible to minimize repulsions.

Molecule	Chemical Formula	Geometry
Methane	\( \mathrm{CH_4} \)	Tetrahedral
Boron Trifluoride	\( \mathrm{BF_3} \)	Trigonal Planar
Ammonia	\( \mathrm{NH_3} \)	Trigonal Pyramidal
Water	\( \mathrm{H_2O} \)	Bent

Analyzing each molecule:

• \( \mathrm{CH_4} \): Methane has a central carbon atom with four hydrogen atoms, leading to a tetrahedral geometry due to four bonding pairs.
• \( \mathrm{BF_3} \): Boron trifluoride has a central boron atom with three fluorine atoms. The three bonding pairs and absence of lone pairs on boron result in a trigonal planar shape.
• \( \mathrm{NH_3} \): Ammonia has a central nitrogen atom with three hydrogen atoms and one lone pair of electrons, leading to a trigonal pyramidal shape.
• \( \mathrm{H_2O} \): Water has a bent shape because of two hydrogen atoms attached to oxygen and two lone pairs on the oxygen.

Thus, the molecule with a trigonal planar shape is \( \mathrm{BF_3} \).

Answer 2

Step 1: Understand molecular geometry 
- \( \mathrm{CH_4} \): tetrahedral shape (4 bonds, no lone pairs)
- \( \mathrm{BF_3} \): 3 bonds, no lone pairs, trigonal planar
- \( \mathrm{NH_3} \): trigonal pyramidal (3 bonds + 1 lone pair)
- \( \mathrm{H_2O} \): bent or angular (2 bonds + 2 lone pairs)

Step 2: Conclusion 
Only \( \mathrm{BF_3} \) has trigonal planar geometry.