<P> Across the lanthanide series, electrons are added to the 4f shell . This first f shell is inside the full 5s and 5p shells (as well as the 6s shell in the neutral atom); the 4f shell is well - localized near the atomic nucleus and has little effect on chemical bonding . The decrease in atomic and ionic radii does affect their chemistry, however . Without the lanthanide contraction, a chemical separation of lanthanides would be extremely difficult . However, this contraction makes the chemical separation of period 5 and period 6 transition metals of the same group rather difficult . </P> <P> There is a general trend of increasing Vickers hardness, Brinell hardness, density and melting point from lanthanum to lutetium (with europium and ytterbium being the most notable exceptions; in the metallic state, they are divalent rather than trivalent). Lutetium is the hardest and densest lanthanide and has the highest melting point . </P> <Table> <Tr> <Td> Element </Td> <Td> Lanthanum </Td> <Td> Cerium </Td> <Td> Praseodymium </Td> <Td> Neodymium </Td> <Td> Promethium </Td> <Td> Samarium </Td> <Td> Europium </Td> <Td> Gadolinium </Td> <Td> Terbium </Td> <Td> Dysprosium </Td> <Td> Holmium </Td> <Td> Erbium </Td> <Td> Thulium </Td> <Td> Ytterbium </Td> <Td> Lutetium </Td> </Tr> <Tr> <Td> Vickers hardness (MPa) </Td> <Td> 491 </Td> <Td> 270 </Td> <Td> 400 </Td> <Td> 343 </Td> <Td>? </Td> <Td> 412 </Td> <Td> 167 </Td> <Td> 570 </Td> <Td> 863 </Td> <Td> 540 </Td> <Td> 481 </Td> <Td> 589 </Td> <Td> 520 </Td> <Td> 206 </Td> <Td> 1160 </Td> </Tr> <Tr> <Td> Brinell hardness (MPa) </Td> <Td> 363 </Td> <Td> 412 </Td> <Td> 481 </Td> <Td> 265 </Td> <Td>? </Td> <Td> 441 </Td> <Td>? </Td> <Td>? </Td> <Td> 677 </Td> <Td> 500 </Td> <Td> 746 </Td> <Td> 814 </Td> <Td> 471 </Td> <Td> 343 </Td> <Td> 893 </Td> </Tr> <Tr> <Td> Density (g / cm) </Td> <Td> 6.162 </Td> <Td> 6.770 </Td> <Td> 6.77 </Td> <Td> 7.01 </Td> <Td> 7.26 </Td> <Td> 7.52 </Td> <Td> 5.264 </Td> <Td> 7.90 </Td> <Td> 8.23 </Td> <Td> 8.540 </Td> <Td> 8.79 </Td> <Td> 9.066 </Td> <Td> 9.32 </Td> <Td> 6.90 </Td> <Td> 9.841 </Td> </Tr> <Tr> <Td> Melting point (K) </Td> <Td> 1193 </Td> <Td> 1068 </Td> <Td> 1208 </Td> <Td> 1297 </Td> <Td> 1315 </Td> <Td> 1345 </Td> <Td> 1099 </Td> <Td> 1585 </Td> <Td> 1629 </Td> <Td> 1680 </Td> <Td> 1734 </Td> <Td> 1802 </Td> <Td> 1818 </Td> <Td> 1097 </Td> <Td> 1925 </Td> </Tr> <Tr> <Td> Atomic radius (pm) </Td> <Td> 187 </Td> <Td> 181.8 </Td> <Td> 182 </Td> <Td> 181 </Td> <Td> 183 </Td> <Td> 180 </Td> <Td> 180 </Td> <Td> 180 </Td> <Td> 177 </Td> <Td> 178 </Td> <Td> 176 </Td> <Td> 176 </Td> <Td> 176 </Td> <Td> 176 </Td> <Td> 174 </Td> </Tr> </Table> <Tr> <Td> Element </Td> <Td> Lanthanum </Td> <Td> Cerium </Td> <Td> Praseodymium </Td> <Td> Neodymium </Td> <Td> Promethium </Td> <Td> Samarium </Td> <Td> Europium </Td> <Td> Gadolinium </Td> <Td> Terbium </Td> <Td> Dysprosium </Td> <Td> Holmium </Td> <Td> Erbium </Td> <Td> Thulium </Td> <Td> Ytterbium </Td> <Td> Lutetium </Td> </Tr>

Which of the following elements is part of the lanthanide series i.e. an inner transition metal