Problem 12 - Chemistry The Central Science

Question

(a) Methane \(\left(\mathrm{CH}_{4}\right)\) and the perchlorate ion \(\left(\mathrm{ClO}_{4}^{-}\right)\) are both described as tetrahedral. What does this indicate about their bond angles? (b) The \(\mathrm{NH}_{3}\) molecule is trigonal pyramidal, while \(\mathrm{BF}_{3}\) is trigonal planar. Which of these molecules is flat?

Step-by-Step Solution

Answer

(a) The bond angles in both methane (CH₄) and the perchlorate ion (ClO₄⁻) are 109.5°, because they have a tetrahedral geometry. (b) The BF₃ molecule is flat, while the NH₃ molecule is not, as BF₃ has a trigonal planar geometry and NH₃ has a trigonal pyramidal geometry due to the presence of a lone pair of electrons.

Step 1: Tetrahedral Geometry

Both CH₄ and ClO₄⁻ are described as having tetrahedral molecular geometries. This means that they have central atoms (C in CH₄ and Cl in ClO₄⁻) surrounded by four bonded atoms, all positioned at the vertices of a regular tetrahedron.

Step 2: Tetrahedral Bond Angles

The bond angles of a tetrahedral molecule are always 109.5°. Since both CH₄ and ClO₄⁻ are tetrahedral, the bond angles in both of these molecules will be 109.5°. Answer to (a): The bond angles in both methane (CH₄) and the perchlorate ion (ClO₄⁻) are 109.5°.

Step 3: Understanding Trigonal Pyramidal and Trigonal Planar Geometry

NH₃ has a trigonal pyramidal geometry, which means it has a central atom (N) surrounded by three bonded atoms (H) and a lone pair of electrons. The atoms are positioned at the vertices of a triangle, while the lone pair occupies the position above the central atom, forming a pyramid shape. BF₃, on the other hand, has a trigonal planar geometry - it has a central atom (B) surrounded by three bonded atoms (F) arranged at the vertices of an equilateral triangle, with all atoms lying in the same plane.

Step 4: Determine Flat Molecules

From the geometries described in Step 3, we can see that NH₃ is not flat because it has a pyramidal shape resulting from the presence of the lone pair of electrons. Conversely, BF₃ is considered flat because the central atom and all the bonded atoms lie in the same plane. Answer to (b): The BF₃ molecule is flat, while the NH₃ molecule is not.