In the molecule dihydrogen sulfide, the four electron groups around the sulfur atom are arranged in a tetrahedral geometry. However the shape of the molecule is bent. Why is the shape of dihydrogen sulfide different from the name of the electron group geometry?
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Basically, electron pair geometry is simply placing the correct number of electrons around the central atom – but only the electrons, not other atoms. In this case, the central atom is sulfur, and there are 8 electrons to be distributed for the bonding of dihydrogen sulfide. So if we use electron pair geometry, we’ll see that sulfur is going to be surrounded by those 8 electrons – and yes, it would look tetrahedral. However, once you attach the hydrogen atoms to sulfur, it is no longer tetrahedral. This is where molecular geometry comes in:
If you look at the family on the periodic table which sulfur is from (6A), you’ll notice that is in the same family as oxygen. You may know that oxygen’s shape is bent, or V-shaped, as you can observe from the shape of a water molecule. Every element in the same family as oxygen will have the same shape – bent.
(I tried to find a link to something that could show you the relative shapes of different elements, but I had no luck. )
Once we attach the hydrogen atoms to the central sulfur atom, the hydrogen’s takes the place of 4 of the 8 electrons. This means that the 4 electrons left are arranged around sulfur to fill its octet. These lone pairs of electrons around sulfur further dictate the shape of the molecule, as they cause sulfur to pull on the hydrogen atoms, forcing the shape to be bent.
So, to recap: Electron pair geometry is simply placing electrons around a central atom, and does not dictate shape; Molecular geometry is placing other atoms around the central atom, and determining the shape of the molecule, based on lone pairs of electrons and the representative shape of the element.
Hopefully that answers your question!