<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> <P> A period is a horizontal row in the periodic table . Although groups generally have more significant periodic trends, there are regions where horizontal trends are more significant than vertical group trends, such as the f - block, where the lanthanides and actinides form two substantial horizontal series of elements . </P> <P> Elements in the same period show trends in atomic radius, ionization energy, electron affinity, and electronegativity . Moving left to right across a period, atomic radius usually decreases . This occurs because each successive element has an added proton and electron, which causes the electron to be drawn closer to the nucleus . This decrease in atomic radius also causes the ionization energy to increase when moving from left to right across a period . The more tightly bound an element is, the more energy is required to remove an electron . Electronegativity increases in the same manner as ionization energy because of the pull exerted on the electrons by the nucleus . Electron affinity also shows a slight trend across a period . Metals (left side of a period) generally have a lower electron affinity than nonmetals (right side of a period), with the exception of the noble gases . </P>

This is a horizontal arrangement of elements in the periodic table