<Ul> <Li> </Li> <Li> </Li> <Li> </Li> </Ul> <Table> <Tr> <Td_colspan="2"> IUPAC group </Td> <Th> </Th> <Th> </Th> <Th> </Th> <Th> </Th> <Th> </Th> <Th> 5 </Th> <Th> 6 </Th> <Th> 7 </Th> <Th> 8 </Th> <Th> 9 </Th> <Th> 10 </Th> <Th> 11 </Th> <Th> 12 </Th> <Th> 13 </Th> <Th> 14 </Th> <Th> 15 </Th> <Th> 16 </Th> <Th> 17 </Th> <Th> 18 </Th> </Tr> <Tr> <Td_colspan="2"> Mendeleev (I--VIII) </Td> <Td> IA </Td> <Td> II A </Td> <Td> III B </Td> <Td> </Td> <Td> IV B </Td> <Td> VB </Td> <Td> VI B </Td> <Td> VII B </Td> <Td_colspan="3"> VIII B </Td> <Td> IB </Td> <Td> II B </Td> <Td> III B </Td> <Td> IV B </Td> <Td> VB </Td> <Td> VI B </Td> <Td> VII B </Td> <Td> </Td> </Tr> <Tr> <Td_colspan="2"> CAS (US, A-B-A) </Td> <Td> IA </Td> <Td> IIA </Td> <Td> IIIB </Td> <Td> </Td> <Td> IVB </Td> <Td> VB </Td> <Td> VIB </Td> <Td> VIIB </Td> <Td_colspan="3"> VIIIB </Td> <Td> IB </Td> <Td> IIB </Td> <Td> IIIA </Td> <Td> IVA </Td> <Td> VA </Td> <Td> VIA </Td> <Td> VIIA </Td> <Td> VIIIA </Td> </Tr> <Tr> <Td_colspan="2"> old IUPAC (Europe, A-B) </Td> <Td> IA </Td> <Td> IIA </Td> <Td> IIIA </Td> <Td> </Td> <Td> IVA </Td> <Td> VA </Td> <Td> VIA </Td> <Td> VIIA </Td> <Td_colspan="3"> VIII B </Td> <Td> IB </Td> <Td> IIB </Td> <Td> IIIB </Td> <Td> IVB </Td> <Td> VB </Td> <Td> VIB </Td> <Td> VIIB </Td> <Td> 0 </Td> </Tr> <Tr> <Td_colspan="2"> Trivial name </Td> <Td> Alkali metals </Td> <Td> Alkaline earth metals </Td> <Td> </Td> <Td> </Td> <Td> </Td> <Td> </Td> <Td> </Td> <Td> </Td> <Td> </Td> <Td> </Td> <Td> </Td> <Td> Coinage metals </Td> <Td> </Td> <Td> Triels </Td> <Td> Tetrels </Td> <Td> Pnictogens </Td> <Td> Chalcogens </Td> <Td> Halogens </Td> <Td> Noble gases </Td> </Tr> <Tr> <Td_colspan="2"> Name by element </Td> <Td> Lithium group </Td> <Td> Beryllium group </Td> <Td> Scandium group </Td> <Td> </Td> <Td> Titanium group </Td> <Td> Vanadium group </Td> <Td> Chromium group </Td> <Td> Manganese group </Td> <Td> Iron group </Td> <Td> Cobalt group </Td> <Td> Nickel group </Td> <Td> Copper group </Td> <Td> Zinc group </Td> <Td> Boron group </Td> <Td> Carbon group </Td> <Td> Nitrogen group </Td> <Td> Oxygen group </Td> <Td> Fluorine group </Td> <Td> Helium or Neon group </Td> </Tr> <Tr> <Td_colspan="2"> Period 1 </Td> <Td> </Td> <Td> </Td> <Td> </Td> <Td> </Td> <Td> </Td> <Td> </Td> <Td> </Td> <Td> </Td> <Td> </Td> <Td> </Td> <Td> </Td> <Td> </Td> <Td> </Td> <Td> </Td> <Td> </Td> <Td> </Td> <Td> </Td> <Td> </Td> <Td> He </Td> </Tr> <Tr> <Td_colspan="2"> Period 2 </Td> <Td> Li </Td> <Td> Be </Td> <Td> </Td> <Td> </Td> <Td> </Td> <Td> </Td> <Td> </Td> <Td> </Td> <Td> </Td> <Td> </Td> <Td> </Td> <Td> </Td> <Td> </Td> <Td> </Td> <Td> </Td> <Td> </Td> <Td> </Td> <Td> </Td> <Td> Ne </Td> </Tr> <Tr> <Td_colspan="2"> Period 3 </Td> <Td> Na </Td> <Td> Mg </Td> <Td> </Td> <Td> </Td> <Td> </Td> <Td> </Td> <Td> </Td> <Td> </Td> <Td> </Td> <Td> </Td> <Td> </Td> <Td> </Td> <Td> </Td> <Td> Al </Td> <Td> Si </Td> <Td> </Td> <Td> </Td> <Td> Cl </Td> <Td> Ar </Td> </Tr> <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> </Table> <Tr> <Td_colspan="2"> IUPAC group </Td> <Th> </Th> <Th> </Th> <Th> </Th> <Th> </Th> <Th> </Th> <Th> 5 </Th> <Th> 6 </Th> <Th> 7 </Th> <Th> 8 </Th> <Th> 9 </Th> <Th> 10 </Th> <Th> 11 </Th> <Th> 12 </Th> <Th> 13 </Th> <Th> 14 </Th> <Th> 15 </Th> <Th> 16 </Th> <Th> 17 </Th> <Th> 18 </Th> </Tr> <Tr> <Td_colspan="2"> Mendeleev (I--VIII) </Td> <Td> IA </Td> <Td> II A </Td> <Td> III B </Td> <Td> </Td> <Td> IV B </Td> <Td> VB </Td> <Td> VI B </Td> <Td> VII B </Td> <Td_colspan="3"> VIII B </Td> <Td> IB </Td> <Td> II B </Td> <Td> III B </Td> <Td> IV B </Td> <Td> VB </Td> <Td> VI B </Td> <Td> VII B </Td> <Td> </Td> </Tr>

Why are there 18 groups in the periodic table