Answer:
The ideal bond angle is 120°, but the real bond angle of bent molecules is usually a few degrees less.
Explanation:
The bonds of a trigonal planar AX₃ molecule point toward the corners of an equilateral triangle.
They trisect a circle into three angles of 120° each.
An example is BF₃ (Figure 1). It is perfectly symmetrical, so the F-B-F bond angles are exactly 120°.
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Now, consider ozone (Figure 2).
The two O-O bonds are equivalent because of resonance.
There are two bonding groups and a lone pair, for a total of three electron groups.
The electron geometry is trigonal planar, and the molecular shape is bent.
However, the lone pair occupies more space than the bonding electrons.
It repels the bolding electrons, so the O-O-O bond angle is 116°, slightly less than the ideal angle of 120°.
