Which of the following best resembles the molecular structure of a hydrogen sulfide molecule?
Hydrogen sulfide has the symbol H₂S. The element sulfur could be found in group 16 of the periodic table, sometimes called group six. Sulfur is just below oxygen. So it’s chemically quite similar. As such, hydrogen sulfide is similar to water, H₂O. Sulfur and oxygen atoms are both much larger than hydrogen atoms. Hydrogen is only in period one while oxygen is in period two. And sulfur is in period three.
Using this information, we can start to interrogate the diagrams we’ve been given. The larger circles indicate sulfur. The smaller circles indicate hydrogen. And the lines indicate bonds between them. A good starting point to understand the molecular structure of hydrogen sulfide is to draw the Lewis structure. We place the sulfur in the middle because it can form more bonds than hydrogen can and then place the hydrogens on either side. As sulfur is in group 16, we expect an atom of sulfur to contribute six valence electrons. And we know that hydrogens each contribute one. This gives us eight electrons.
We use four electrons forming single bonds between the sulfur and the hydrogens. And we use the remaining four to complete sulfur’s octet, creating two lone pairs. This means we have two bonding pairs and two lone pairs. This means we end up with a bent geometry, where the lone pairs and the bonding pairs try and get this far away from each other as they can, minimizing repulsions. Based on the structure, we can eliminate option A. Hydrogens can’t form more than one bond. So we can’t have hydrogen as part of a chain. And we know that hydrogen sulfide has a bent geometry. So it can’t have a bond angle of 180 degrees. That just leaves the option of 90 degrees, 60 degrees, and 30 degrees.
In water, the bond angle between the two hydrogens is 104.5 degrees. This is slightly less than the angle you’d expect from a perfect tetrahedron of 109.5 degrees. The bond angle is smaller because the lone pairs crush the hydrogens together a little bit more. Lone pairs are more repulsive than bonding pairs. So what does that mean for sulfur? Well, we still have the same lone pair-bonding pair repulsion. But a sulfur atom is considerably bigger than an oxygen atom. So the hydrogens are further away and repel each other less. So we expect the bond angle between the hydrogens to be even less than what we see in water. But angles of 30 degrees and 60 degrees would lead to considerable increases in hydrogen-hydrogen repulsion.
So the structure that best resembles the molecular structure of a hydrogen sulfide molecule is the one where we have the two hydrogen atoms bound to the sulfur atom at an angle of 90 degrees. And the measured value for the actual angle is about 92 degrees. But interestingly, if we substitute the hydrogens for a larger atom, like chlorine, the angle gets much closer to what we’d expect for a tetrahedron. So a part of what makes the bond angle in hydrogen sulfide so small is that the hydrogen atoms are tiny, relatively speaking.