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.