Predicting molecular shape without drawing lewis structure

[Immutef]

Is their a method for predicting the molecular shape of molecules without drawing the Lewis structure?

I am preparing for an ACS exam and would like to try to save some time on these problems. I think I could determine the steric number by counting the total electrons available, and subtracting the product of the number of bonded molecules x 8. The remaining electrons would be counted as lone pairs.

I don't think this would be accurate all the time because it does not account for double, triple bonds etc.

Is their "quick" way to predict molecular shape that is more reliable.

Thanks

 

[DDTea]

Simply put: yes, but with a good bit of experience which it sounds like you don't have (no offense intended, just a reality check). I have a sort of "database" of molecules with common shapes in my head which I can make quick comparisons to when I'm encountering a new molecule. That's usually my starting point.

For standardized tests, though, my advice is to do it by the book. Writing things down helps you avoid mistakes, which are incredibly easy to make on standardized tests.

 

[Immutef]

Simply put: yes, but with a good bit of experience which it sounds like you don't have (no offense intended, just a reality check). I have a sort of "database" of molecules with common shapes in my head which I can make quick comparisons to when I'm encountering a new molecule. That's usually my starting point.

For standardized tests, though, my advice is to do it by the book. Writing things down helps you avoid mistakes, which are incredibly easy to make on standardized tests.

I'm a first year student taking general chemistry 1

 

[DDTea]

And I've completed my degree and molecular geometry still often surprises me unless I approach it in a systematic way. It can get interesting with transition metals especially.

There's a saying from the Marine corps: slow is smooth and smooth is fast. What it means is that when you're learning something, start slow and pay attention to detail. With time and practice, speed will come.

 

[LudusRex]

for your question. There are indeed methods for predicting molecular shape without drawing a Lewis structure. One commonly used method is the Valence Shell Electron Pair Repulsion (VSEPR) theory. This theory takes into account the number of electron pairs around a central atom and the repulsion between these pairs to predict the molecular shape. The steric number, as you mentioned, can be used to determine the number of electron pairs. However, it is important to note that this method is not always accurate, especially for molecules with multiple bonds. Other factors such as the electronegativity of the atoms and the presence of lone pairs also play a role in determining the molecular shape. A more reliable method would be to use computational methods such as molecular orbital theory or density functional theory. These methods use mathematical equations and algorithms to predict the molecular shape based on the electronic structure of the molecule. However, they may require more time and resources to use compared to the VSEPR theory. Ultimately, it is best to use a combination of methods to predict molecular shape, including drawing a Lewis structure as a starting point. Good luck on your exam!