# Bond sizes in organic chemistry

1. Jan 15, 2005

### decamij

I know that a double bond has the two atoms closer to each other than a single bond. I have to build an organic acid model for chemistry and would like to know:
- Is there a ratio between the lengths of C-C bonds to C=C bonds?
- Are C-H bonds closer together than C-C bonds?
- Are C=O bonds closer together than C=C bonds?
- If i'm building a model out of styrofoam balls and sticks, would C and O atoms be essentially the same size (i know O has a smaller atomic radius than C, but is it significant) --> i intend on using a colour code (black - C, red - O)

The first question is my top priority. Any help would make a huge difference - thanx!

2. Jan 15, 2005

### dextercioby

From what i know of,it's roughly 2.That is the simple bond is twice as "long" as the double one.

No,it's the nature of the bond that is essential:in this simple sigma bonds for both situations...

I see no reason why they should be different...One sigma,one pi,double bond...

No,there is no significant radius difference.You can put the O atom a bit bigger,but not significantly...

Daniel.

3. Jan 15, 2005

### Gokul43201

Staff Emeritus
Dexter, that's not right !
These are roughly the bond lengths of the various combinations that you might need (in picometers) :

H--N 101
H--O 96
O--O 148

C--C 154
C=C 134
C$\equiv$C 120

c--H 109
C--N 147
C--O 143
C--Cl 177
C--Br 194

Empirical atomic radii of various atoms (in pm) are roughly ("atomic radius" is as such, a very vague term as it can be defined and measured in different ways) :

H 25
C 70
N 65
O 60
Cl 100
Br 115

Since the bond length is defined as the internuclear distance (ie : the sum of the covalent radii, in the case of a single bond) it does depend on the size of the atoms involved.

If you are using styrofoam balls and sticks, remember that the length of stick exposed is given by :
$$length~exposed = bond~length - (radius1 + radius2)$$.
If the sum of the radii equals (or exceeds) the bond length, then no part of the stick will be exposed, and the balls will have to be touching each other. In fact, if the bond legth is greater than the sum of the radii (eg : C=C), you will have to flatten part of the C atoms (the balls) to make the distance correct.

Yes, O is smaller. See the numbers above, for the atomic radii.

Another way of making such models involves using small balls (all of the same size, but different colors) to represent the atoms, but leave out information about the atomic radii. By making the balls small compared to the length of the sticks, you only have to worry about bond lengths, which will now be equal to the lengths of the sticks.

Last edited: Jan 15, 2005
4. Jan 15, 2005

### Gokul43201

Staff Emeritus
I forgot to include

C=O 127 pm