Chemistry · Ch 4 — The d- and f-Block Elements

The Actinoids

4.6

The Actinoids

The actinoids are the block of fourteen elements that follow actinium, running from thorium (Th, $Z=90$ ) to lawrencium (Lr, $Z=103$ ). Together with actinium they make up the second row of the f-block, positioned below the lanthanoids in the periodic table. Their names, symbols, and a set of key properties — electronic configurations, ionic configurations, and ionic radii of the $M^{3+}$ and $M^{4+}$ states — are collected for reference (see the accompanying properties table).

Table 4.10Some Properties of Actinium and Actinoids

Atomic Number	Name	Symbol	Config. M	Config. $M^{3+}$	Config. $M^{4+}$	Radius $M^{3+}$ /pm	Radius $M^{4+}$ /pm
89	Actinium	Ac	$6d^17s^2$	$5f^0$		111
90	Thorium	Th	$6d^27s^2$	$5f^1$	$5f^0$		99
91	Protactinium	Pa	$5f^26d^17s^2$	$5f^2$	$5f^1$		96
92	Uranium	U	$5f^36d^17s^2$	$5f^3$	$5f^2$	103	93
93	Neptunium	Np	$5f^46d^17s^2$	$5f^4$	$5f^3$	101	92
94	Plutonium	Pu	$5f^67s^2$	$5f^5$	$5f^4$	100	90
95	Americium	Am	$5f^77s^2$	$5f^6$	$5f^5$	99	89
96	Curium	Cm	$5f^76d^17s^2$	$5f^7$	$5f^6$	99	88
97	Berkelium	Bk	$5f^97s^2$	$5f^8$	$5f^7$	98	87
98	Californium	Cf	$5f^{10}7s^2$	$5f^9$	$5f^8$	98	86
99	Einsteinium	Es	$5f^{11}7s^2$	$5f^{10}$	$5f^9$	—	—
100	Fermium	Fm	$5f^{12}7s^2$	$5f^{11}$	$5f^{10}$	—	—
101	Mendelevium	Md	$5f^{13}7s^2$	$5f^{12}$	$5f^{11}$	—	—
102	Nobelium	No	$5f^{14}7s^2$	$5f^{13}$	$5f^{12}$	—	—
103	Lawrencium	Lr	$5f^{14}6d^17s^2$	$5f^{14}$	$5f^{13}$	—	—

Every member of this series is radioactive, and this single fact shapes almost everything about how the actinoids are studied and used. The half-lives are not uniform across the series: the earlier actinoids (such as thorium and uranium) have comparatively long half-lives, while the later members decay far more quickly — lawrencium's half-life is only around three minutes. Because of this rapid decay, the heaviest actinoids can only ever be made in nanogram quantities, which makes their isolation and chemical study considerably more difficult than that of the lighter elements in the series. …

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