<Tr> <Td_colspan="2"> Period 4 </Td> <Td> </Td> <Td> Ca </Td> <Td> Sc </Td> <Td> </Td> <Td> Ti </Td> <Td> V </Td> <Td> Cr </Td> <Td> Mn </Td> <Td> Fe </Td> <Td> Co </Td> <Td> Ni </Td> <Td> Cu </Td> <Td> Zn </Td> <Td> Ga </Td> <Td> Ge </Td> <Td> As </Td> <Td> Se </Td> <Td> Br </Td> <Td> Kr </Td> </Tr> <Tr> <Td_colspan="2"> Period 5 </Td> <Td> Rb </Td> <Td> Sr </Td> <Td> Y </Td> <Td> </Td> <Td> Zr </Td> <Td> Nb </Td> <Td> Mo </Td> <Td> Tc </Td> <Td> Ru </Td> <Td> Rh </Td> <Td> Pd </Td> <Td> Ag </Td> <Td> Cd </Td> <Td> In </Td> <Td> Sn </Td> <Td> Sb </Td> <Td> Te </Td> <Td> </Td> <Td> Xe </Td> </Tr> <Tr> <Td_colspan="2"> Period 6 </Td> <Td> Cs </Td> <Td> Ba </Td> <Td> La </Td> <Td> Ce--Lu </Td> <Td> Hf </Td> <Td> Ta </Td> <Td> </Td> <Td> Re </Td> <Td> Os </Td> <Td> Ir </Td> <Td> Pt </Td> <Td> Au </Td> <Td> Hg </Td> <Td> Tl </Td> <Td> Pb </Td> <Td> Bi </Td> <Td> Po </Td> <Td> At </Td> <Td> Rn </Td> </Tr> <Tr> <Td_colspan="2"> Period 7 </Td> <Td> Fr </Td> <Td> Ra </Td> <Td> Ac </Td> <Td> Th--Lr </Td> <Td> Rf </Td> <Td> Db </Td> <Td> Sg </Td> <Td> Bh </Td> <Td> Hs </Td> <Td> Mt </Td> <Td> Ds </Td> <Td> Rg </Td> <Td> Cn </Td> <Td> Nh </Td> <Td> Fl </Td> <Td> Mc </Td> <Td> Lv </Td> <Td> Ts </Td> <Td> Og </Td> </Tr>

Which two letters represent elements in the same group