Which letter replaces the question mark?
Each bond gives the number of letters in common and in the same position for chemical elements given by their atomic number.
For example 1~2 = Hydrogen ~ Helium have 1 bond in common
while 1 ~ 7 = Hydrogen ~ Nitrogen have 5 - Thanks to David Starkey
So number 6 is correct. It has the correct number of bonds since it has one letter in common position with each of the linked atomic numbered elements
So thanks to Ian McDonald the number 6 can be represented by letter C for Carbon
Where can #10 be placed in this diagram?
Not a double bond from one node to another here. But where is L?
Each link gives the letters in common position with letters given by the phonetic alphabet - with version A for Alpha and J for Juliett which may vary.
So L is for Lima and the only letter it has a common one with is Kilo so L has one bond to K
This answer was found by user who gave it in an earlier comment - thank you! - I have missed your user name at moment.