Rules to find the atomic no. of middle elements are as follows:
1)Take odd no. of elements such as 3 elements or 5  elements etc.
2)The first and the last one's atomic no. will be given so to find the middle one's atomic no. do as follows 
i)If 3 elements are given suppose X,Y,Z where the atomic no. of X=16 & Z=18 then y's atomic no. will be 
Y=16+18/2
=>Y=17
ii)If 5 elements suppose V,W,X,Y,Z where V=32 & Z=36 then do as follows
here middle one is X
X=32+36/2
=>X=34
then W=32+34/2
W=33
Similarly Y=34+36/2
Y=35
Hence these are the rules you must follow to find it out 
Note:-Do not compare it with the laws of triads
 Discovered by,
Adityam Ghosh And Pratik Banerjee