π Understanding Bond Angles in VSEPR Theory
Valence Shell Electron Pair Repulsion (VSEPR) theory helps us predict the shapes of molecules based on the idea that electron pairs surrounding a central atom repel each other and want to be as far apart as possible. This repulsion affects the bond angles.
π Linear Structures
- βοΈ Definition: A linear molecule has atoms arranged in a straight line.
- π Bond Angle: The bond angle in a linear molecule is always $180^{\circ}$.
- π§ͺ Example: Beryllium chloride ($BeCl_2$) and carbon dioxide ($CO_2$). In $CO_2$, the central carbon atom is bonded to two oxygen atoms, and there are no lone pairs on the carbon, resulting in a linear shape.
β¨ Trigonal Planar Structures
- βοΈ Definition: A trigonal planar molecule has three atoms bonded to a central atom, all lying in the same plane.
- π Bond Angle: Ideally, the bond angles are $120^{\circ}$.
- π§ͺ Example: Boron trifluoride ($BF_3$). The boron atom is bonded to three fluorine atoms. Note that if there are lone pairs, the angles can deviate from the ideal.
- π‘ Note: If one of the bonded atoms is replaced by a lone pair of electrons (resulting in a bent shape), the bond angle will be slightly less than $120^{\circ}$ due to the greater repulsion from the lone pair.
β°οΈ Tetrahedral Structures
- βοΈ Definition: A tetrahedral molecule has a central atom bonded to four other atoms, forming a three-dimensional tetrahedron shape.
- π Bond Angle: The bond angles in a perfect tetrahedron are approximately $109.5^{\circ}$.
- π§ͺ Example: Methane ($CH_4$). The carbon atom is bonded to four hydrogen atoms.
- π‘ Note: If lone pairs are present, the bond angles will be smaller than $109.5^{\circ}$ because lone pair-bond pair repulsions are stronger than bond pair-bond pair repulsions. For example, in ammonia ($NH_3$), the bond angle is about $107^{\circ}$ because of the lone pair on nitrogen. In water ($H_2O$), the bond angle is about $104.5^{\circ}$ because of the two lone pairs on oxygen.
π Summary Table
| Structure |
Bond Angle(s) |
Example |
| Linear |
$180^{\circ}$ |
$CO_2$ |
| Trigonal Planar |
$120^{\circ}$ |
$BF_3$ |
| Tetrahedral |
$109.5^{\circ}$ |
$CH_4$ |
π§ͺ Practice Quiz
- β What is the bond angle in $BeCl_2$?
- β What is the ideal bond angle in $BF_3$?
- β What is the approximate bond angle in $CH_4$?
- β Why is the bond angle in $NH_3$ less than $109.5^{\circ}$?
- β What is the molecular geometry of $CO_2$?
- β What is the molecular geometry of $CH_4$?
- β How does the presence of lone pairs affect bond angles?