Final answer:
Lone pair electrons lead to a repulsion that can cause a molecule to have a bent or angular shape rather than a typical geometric shape like tetrahedral. The VSEPR theory explains this by stating that electron pairs repel each other, affecting molecular geometry. Double or triple bonds count as a single electron group in determining shape.
Step-by-step explanation:
The presence of lone pair electrons significantly affects the three-dimensional shape of a molecule. According to the valence shell electron pair repulsion (VSEPR) theory, electron pairs around a central atom, including bond pairs and lone pairs, repel each other to maximize the distance between them. This repulsion influences the molecular geometry, which is the arrangement of atoms in a molecule. For example, in a molecule with four electron groups around the central atom, if one of those groups is a lone pair, the geometry of the electron groups is tetrahedral. However, the absence of a bonded atom causes the molecule to take on a bent or angular shape rather than a true tetrahedral shape. This phenomenon is evident in water (H2O), where two lone pairs on the oxygen atom result in a bent molecular shape.
Double or triple bonds are treated as a single electron group in VSEPR theory. Therefore, in a molecule like CH2O, which has a double bond, the shape is determined by the total number of electron groups and not by the multiple bonds themselves. Furthermore, any molecule with only two atoms, such as BeH2 or CO2, is linear due to the two electron groups being oriented 180° apart to minimize repulsion, which translates into a linear arrangement of atoms. In summary, lone pairs cause a repulsion that often leads to a bent or angular molecular shape, while molecules without lone pairs and with only two electron groups take on a linear geometry.