Final answer:
The HOH molecule is bent due to two lone pairs of electrons on the central oxygen causing repulsion which shapes the molecule into a bent geometry. The HBeH molecule, lacking lone pairs on the central beryllium atom, is linear because the two electron pairs shared with hydrogen are as far apart as possible without any lone pair repulsion.
Step-by-step explanation:
The reason why the HOH molecule is bent while the HBeH molecule is linear has to do with the presence of lone pairs and electron-pair geometry. In the HOH molecule, also known as water, there are two hydrogen atoms bonded to a central oxygen atom which also possesses two lone pairs of electrons.
The electron-pair geometry aims to minimize electron-pair repulsion, which in the case of water leads to a tetrahedral arrangement. However, since there are no atoms at the locations of the two lone pairs, the molecular shape is actually bent.
On the other hand, the HBeH molecule, which includes beryllium (Be), does not have lone pairs as Be has only two electrons to share with hydrogen atoms.
This means that the electron pairs between H and Be will arrange themselves as far apart as possible, resulting in a linear shape since there are no lone pairs to cause repulsion and bend the molecule.