## Writing Bond Angles with Multiple Central Angles

(Polar molecules, Non-polar molecules, etc.)

KatelinTanjuaquio 1L
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### Writing Bond Angles with Multiple Central Angles

I was doing a problem in which it asks to write the bond angles for C2H4. How do we know which atoms to look at? I understand how we look at H-C-H but how do we know to do H-C-C instead of, say C-C?

Michael Torres 4I
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### Re: Writing Bond Angles with Multiple Central Angles

You know you would be unable to do C-C because that doesn't have an angle. That's just a bond. Triangles have angles on their corners because there are lines pointing in two different directions from each corner. If you tried to find the angle of the bond between two atoms you would just get 180 degrees every single time since the bond itself would just be a straight line.

In the particular example you are talking about, the molecular shape is trigonal planar about both carbon central atoms. You would be able to determine which bond angles to calculate simply because these are the only angles which might vary. Although in this example all the bond angles are basically just the same, the angles in a different example might vary. Essentially, what I am trying to say is that you need to find all the different angles in the molecule, but it just so happens in this case that they are all the same.

The configurations H-C-H and H-C-C are really the only two different configurations of atoms in this molecule that exist which may have different bond angles. Therefore, finding the bond angles of both of them should suffice for all bond angles in the molecule itself. (You could also mention C-C-H, but that's really just the same as H-C-C.)

Srikar_Ramshetty 1K
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### Re: Writing Bond Angles with Multiple Central Angles

Great question! I believe Dr. Lavelle had mentioned this and it is a rather simple concept, at least in your example, since to know what bond angle to look at you need 3 atoms. C-C wouldn't be of any interest leaving only H-C-C. I think this can be explained with a mathematical concept, where two points on the same plane can be used to form a line, which would indicate there is no difference in angle between any two points, but also to form an angle in math you would two intersecting lines or 3 points and in this case these would be the 3 atoms.

Hope that helps.