A bond angle is defined as the angle formed between two bonds that originate from the same central atom in a molecule. For instance, when examining a central atom connected to two surrounding atoms, the bond angle is measured from one bond to the other. In cases where a lone pair of electrons is present, it is important to focus solely on the bonds between the central atom and the surrounding atoms, disregarding the lone pair in the angle measurement.

When a central atom has no lone pairs, it exhibits an ideal bond angle, which is the optimal angle that minimizes repulsion between the bonded atoms. This ideal bond angle varies depending on the molecular geometry. However, the presence of one or more lone pairs alters this ideal bond angle. Lone pairs exert a repulsive force that pushes the bonded atoms closer together, resulting in a decrease in the bond angle. For example, in a scenario where the central atom has a lone pair, the bond angle will be smaller compared to a situation where the central atom has no lone pairs.

In summary, the absence of lone pairs allows for the ideal bond angle to be maintained, while the introduction of lone pairs leads to a reduction in the bond angle due to increased repulsion. Understanding this concept is crucial for predicting molecular shapes and angles in chemistry.