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A series of elements in the periodic table, generally considered to range in proton number from cerium (58) to lutetium (71) inclusive. The lanthanoids all have two outer s-electrons (a 6s2 configuration), follow lanthanum, and are classified together because an increasing proton number corresponds to increase in number of 4f electrons. In fact, the 4f and 5d levels are close in energy and the filling is not smooth. The outer electron configurations are as follows: 57 lanthanum (La) 5d16s258 cerium (Ce) 4f5d16s2 (or 4f26s2)59 praseodymium (Pr) 4f36s260 neodymium (Nd) 4f46s261 promethium (Pm) 4f56s262 samarium (Sm) 4f66s263 europium (Eu) 4f76s264 gadolinium (Gd) 4f75d16s265 terbium (Tb) 4f96s266 dysprosium (Dy) 4f106s267 holmium (Ho) 4f116s268 erbium (Er) 4f126s269 thulium (Tm) 4f136s270 ytterbium (Yb) 4f146s271 lutetium (Lu) 4f145d16s2Note that lanthanum itself does not have a 4f electron but it is generally classified with the lanthanoids because of its chemical similarities, as are yttrium (Yt) and scandium (Sc). Scandium, yttrium, and lanthanum are d-block elements; the lanthanoids and actinoids make up the f-block.

57 lanthanum (La) 5d16s2

58 cerium (Ce) 4f5d16s2 (or 4f26s2)

59 praseodymium (Pr) 4f36s2

60 neodymium (Nd) 4f46s2

61 promethium (Pm) 4f56s2

62 samarium (Sm) 4f66s2

63 europium (Eu) 4f76s2

64 gadolinium (Gd) 4f75d16s2

65 terbium (Tb) 4f96s2

66 dysprosium (Dy) 4f106s2

67 holmium (Ho) 4f116s2

68 erbium (Er) 4f126s2

69 thulium (Tm) 4f136s2

70 ytterbium (Yb) 4f146s2

71 lutetium (Lu) 4f145d16s2

The lanthanoids are sometimes simply called the rare earths, although strictly the ‘earths’ are their oxides. Nor are they particularly rare: they occur widely, usually together. All are silvery very reactive metals. The f-electrons do not penetrate to the outer part of the atom and there is no f-orbital participation in bonding (unlike the d-orbitals of the main transition elements) and the elements form few coordination compounds. The main compounds contain M3+ ions. Cerium also has the highly oxidizing Ce4+ state and europium and ytterbium have a M2+ state.

The 4f orbitals in the atoms are not very effective in shielding the outer electrons from the nuclear charge. In going across the series the increasing nuclear charge causes a contraction in the radius of the M3+ ion – from 0.1061 nm in lanthanum to 0.0848 nm in lutetium. This effect, the lanthanoid contraction (or lanthanide contraction), accounts for the similarity between the transition elements zirconium and hafnium.

Subjects: Chemistry.

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