What is the approximate value of the H-C-H bond angles in methane, CH4? Select one: a. 180° b. 109° c. 120° d. 90°
The correct answer and explanation is:
The correct answer is b. 109°.
Explanation:
Methane (CH4) consists of a central carbon atom bonded to four hydrogen atoms. The geometry of methane is based on the tetrahedral arrangement. This arrangement occurs because carbon has four valence electrons, which it shares with four hydrogen atoms, forming four covalent bonds.
In a tetrahedral geometry, the bond angles between any two bonds are approximately 109.5°. This angle is determined by the repulsion between the bonding electron pairs around the central carbon atom, as described by the Valence Shell Electron Pair Repulsion (VSEPR) theory. The theory suggests that electron pairs around a central atom will arrange themselves as far apart as possible to minimize repulsion. In methane, the four pairs of bonding electrons arrange themselves in a way that the bond angles between them are nearly 109.5°.
To visualize this, imagine a tetrahedron, which has four faces that are equilateral triangles. The central carbon atom is at the center of this tetrahedron, and the hydrogen atoms are at the corners. The angle between any two bonds (H-C-H) is the angle between two faces of the tetrahedron, which is 109.5°.
Other geometries with different bond angles include:
- 180° for a linear arrangement (like in CO2, carbon dioxide),
- 120° for a trigonal planar arrangement (like in BF3, boron trifluoride),
- 90° for a square planar arrangement (like in XeF4, xenon tetrafluoride).
Thus, methane’s H-C-H bond angles are closest to 109°, reflecting the tetrahedral shape that minimizes electron pair repulsion.