Chemistry · Ch 8 — The d- and f-Block Elements

Electronic Configurations

8.5.1

Electronic Configurations

The Configuration Pattern

Across the lanthanoid series, the atoms share one common feature: an outer $6s^2$ configuration. What changes steadily from one element to the next is how many electrons sit in the $4f$ level.

Table 4.9Electronic Configurations and Radii of Lanthanum and Lanthanoids

Atomic Number	Name	Symbol	Ln	$Ln^{2+}$	$Ln^{3+}$	$Ln^{4+}$	Radius Ln/pm	Radius $Ln^{3+}$ /pm
57	Lanthanum	La	$5d^16s^2$	$5d^1$	$4f^0$		187	106
58	Cerium	Ce	$4f^15d^16s^2$	$4f^2$	$4f^1$	$4f^0$	183	103
59	Praseodymium	Pr	$4f^36s^2$	$4f^3$	$4f^2$	$4f^1$	182	101
60	Neodymium	Nd	$4f^46s^2$	$4f^4$	$4f^3$	$4f^2$	181	99
61	Promethium	Pm	$4f^56s^2$	$4f^5$	$4f^4$		181	98
62	Samarium	Sm	$4f^66s^2$	$4f^6$	$4f^5$		180	96
63	Europium	Eu	$4f^76s^2$	$4f^7$	$4f^6$		199	95
64	Gadolinium	Gd	$4f^75d^16s^2$	$4f^75d^1$	$4f^7$		180	94
65	Terbium	Tb	$4f^96s^2$	$4f^9$	$4f^8$	$4f^7$	178	92
66	Dysprosium	Dy	$4f^{10}6s^2$	$4f^{10}$	$4f^9$	$4f^8$	177	91
67	Holmium	Ho	$4f^{11}6s^2$	$4f^{11}$	$4f^{10}$		176	89
68	Erbium	Er	$4f^{12}6s^2$	$4f^{12}$	$4f^{11}$		175	88
69	Thulium	Tm	$4f^{13}6s^2$	$4f^{13}$	$4f^{12}$		174	87

Written relative to the $[\text{Xe}]$ core, the general pattern for the neutral atom is

[\text{Xe}]\,4f^{1-14}\,5d^{0-1}\,6s^2

The $5d^{0-1}$ term signals that the filling of the $4f$ subshell is not perfectly smooth — a few members of the series pick up a single $5d$ electron instead of the "expected" extra $4f$ electron, which is why the exact atomic configuration has to be looked up element by element rather than predicted from a simple rule.

The Tripositive Ion Is the Clean Case

While the neutral-atom configurations are somewhat irregular, the picture becomes much simpler once you strip the atoms down to their most stable oxidation state — the $+3$ ion. For every member of the series, the tripositive ion has the configuration

$4f^{n} \qquad (n = 1 \text{ to } 14 \text{, increasing steadily with atomic number})$ …

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