Explain why BeH₂ molecule has a zero dipole moment although the Be–H bonds are polar.
In polyatomic molecules, the dipole moment depends not only on the individual bond dipoles but also on the spatial arrangement of various bonds.
Although the Be–H bonds in BeH₂ are polar due to differences in electronegativity, the molecule has a linear geometry. Because of this linear arrangement, the two equal bond dipoles point in exactly opposite directions.
As a result, the bond dipoles cancel the effect of each other, making the net dipole moment of the BeH₂ molecule zero.
Explanation
The textbook explains that in polyatomic molecules, the net dipole moment depends on both individual bond dipoles and their spatial arrangement. Using the example of BeF2, it states that when two equal bond dipoles point in opposite directions, they cancel each other out, resulting in a zero dipole moment. This same principle applies to BeH2, which has a linear structure where the polar Be-H bond dipoles are equal and opposite, thus summing to zero.