Final answer:
Deviations from idealized bond angles occur due to electron repulsion, orbital hybridization, angle strain, and eclipsing strains, as in the case of the water molecule's bond angle alteration from the expected 109.5° to the observed 104.5° due to hybridized orbitals.
Step-by-step explanation:
Deviations from the idealized bond angle can occur due to factors like electron repulsion, orbital hybridization, and presence of lone pairs of electrons. Ideal bond angles are predicted based on simple overlap of atomic orbitals. For example, in the water molecule (H₂O), the observed bond angle of 104.5° deviates from the expected tetrahedral bond angle of 109.5° due to the hybridization of oxygen's orbitals. Instead of pure p-orbital overlap, we have four hybrid orbitals that include contributions from both s and p orbitals of the oxygen, directing towards the tetrahedron's corners. This also accounts for electron pair repulsion which modifies the angle to less than the ideal. Angle strain and eclipsing strains are additional factors that can cause deviations when double bonds or bulky groups are present. For instance, in certain organic structures, double bonds can repel adjacent single bonds more strongly, leading to smaller or larger bond angles than the ideal geometry predicts.