<Tr> <Td> 7 </Td> <Td> Fr </Td> <Td> Ra 215 </Td> <Td> * * </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> <Tr> <Td> Lanthanides </Td> <Td> * </Td> <Td> La 195 </Td> <Td> Ce 185 </Td> <Td> Pr 185 </Td> <Td> Nd 185 </Td> <Td> Pm 185 </Td> <Td> Sm 185 </Td> <Td> Eu 185 </Td> <Td> Gd 180 </Td> <Td> Tb 175 </Td> <Td> Dy 175 </Td> <Td> Ho 175 </Td> <Td> Er 175 </Td> <Td> Tm 175 </Td> <Td> Yb 175 </Td> <Td> Lu 175 </Td> </Tr> <Tr> <Td> Actinides </Td> <Td> * * </Td> <Td> Ac 195 </Td> <Td> Th 180 </Td> <Td> Pa 180 </Td> <Td> U 175 </Td> <Td> Np 175 </Td> <Td> Pu 175 </Td> <Td> Am 175 </Td> <Td> Cm </Td> <Td> Bk </Td> <Td> Cf </Td> <Td> Es </Td> <Td> Fm </Td> <Td> Md </Td> <Td> No </Td> <Td> Lr </Td> </Tr> <P> The way the atomic radius varies with increasing atomic number can be explained by the arrangement of electrons in shells of fixed capacity . The shells are generally filled in order of increasing radius, since the negatively charged electrons are attracted by the positively charged protons in the nucleus . As the atomic number increases along each row of the periodic table, the additional electrons go into the same outermost shell; whose radius gradually contracts, due to the increasing nuclear charge . In a noble gas, the outermost shell is completely filled; therefore, the additional electron of next alkali metal will go into the next outer shell, accounting for the sudden increase in the atomic radius . </P>

What is the size of a helium atom