500
All of the following are in the same rows/periods of the periodic table EXCEPT...
Beryllium(Be), Oxygen(O), Fluorine (F)
Hydrogen(H), Helium(He)
Sodium(Na),Carbon(C), Chlorine(Cl)
Lithium(Li), Boron(B), Neon(Ne)
This one was a little harder. You needed to remember what elements were in which rows. The correct answer was "Sodium (Na), Carbon(C), Chlorine(Cl)" because carbon is in the second row while sodium and chlorine are in the third row