We certainly understand that hydrogen is smaller than carbon because hydrogen's valence shell is the first shell and carbon's valence shell is the second shell.  What does this mean in terms of bond lengths.

We can define the "covelent radius" of various elements by looking at their inter-atomic distances.  Print this page out and then answer the questions by using the following series of molecules:

The first molecule is hydrogen (H₂).  Confirm that the bond length is 0.70 Angstroms  This would means that the covalent radius is 0.35 Angstroms.

The second molecule is ethane.  Look at the C-C bond.
What is the bond length?
What is the covalent radius for carbon?
The bond length for the C-H bond should be the sum of the radii of carbon and hydrogen, is it?

The third molecule is chloroethane.
What is theC-Cl bond length?
What is the covalent radius for chlorine?

The fourth molecule is iodoethane.
What is theC-I bond length?
What is the covalent radius for iodine?

The fifth molecule is ethene.
What is theC=C bond length?

The sixth molecule is ethyne.
What is the bond length of the carbon/carbon triple bond?