A series of 15 metallic elements in the periodic table with atomic numbers from 57 to 71; the lighter elements in the series resemble lanthanum, hence the name. The elements with their atomic numbers are: lanthanum (La, 57); cerium (Ce, 58); Praseodymium (Pr, 59); neodymium (Nd, 60); promethium (Pm, 61); samarium (Sm, 62); europium (Eu, 63); gadolinium (Gd, 64); terbium (Tb, 65); dysprosium (Dy, 66); holmium (Ho, 67); erbium (Er, 68); thulium (Tm, 69); ytterbium (Yb, 70); lutetium (Lu, 71).