What Is The Molecular Geometry Of SeH2?

H2Se adopts a “bent” structure with a H-Se-H bond angle of 91°.

You can make useful comparision with H-O-H [H2O] and H-SH [H2S]. To explain this, it would be important for you to understand hybridization theory. The bond angle of H-O-H is 105, and that of H- S-H is 92. This would suggest that S is hardly hybridized at all, and in fact using p-orbitals (with little s-characteristics) to form the S-H bonds, whereas O is sp3 hybridized.

To understand why S is not hybridized, we need to understand that the large size of S allows the electron pairs to be far from each other so that the energy incurred from repulsion is not very large and need not be minimized. Since O is a lot smaller, a more stable, less energetic configuration, would have to be made – hence the hybridization.

Each of the molecules H2O, H2Se, H2Te has two lone, non-bonding pairs of electrons. They originate from the atoms O, Se, Te which are in successive rows of the periodic table. As a result the electron pairs are attracted by the nuclei less and less strongly. As a consequence of this lower attraction the lone pairs occupy more space. This in turn causes the bond angle of the H’s to become more acute. This reasoning is just one example of a semi-empirical bonding model called: Valence Shell Electron Pair Repulsion (VSEPR).

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Source(s): retired chem examiner

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The basic geometry is determined by the number of electron pairs whether they are bonding or not. SeH2 has four electron pairs around it, two of which are bonding and two of which are lone pairs. The electron pair geometry is tetrahedral. The molecular geometry is bent and the bond angle is a little less than 109.5 degrees. As it turns out, the bond angle is a lot less than 109.5 degrees, closer to 95 degrees, because of the large size of Se and the spreading out of the delocalized lone pairs which push the bonding pairs closer together.