Question

Explain the position of actinoids in the periodic table.

Answer 1

Position of actinoids in the periodic table: 

i. Actinoids belongs to the third group of periodic table in the seventh period. 

ii. In the periodic table, as we move from one element to another, either from left to right or from top to bottom, the properties exhibit a gradual change. But these fifteen elements are so similar to one another, that they cannot be placed one after the other or one below the other. 

iii. As the fourteen elements i.e. Th(90) to Lr(103) are closely similar to Ac(89), the best place for them is alongwith actinium(89) i.e., third group (3^rd column) and seventh period in the periodic table. 

iv. In case these elements are give different positions in order of their increasing atomic numbers, the symmetry of the periodic table would be disrupted. Due to this reason, the actinoids are placed at the bottom of the periodic table with a reference to the third group in the seventh period i.e., the position of actinium.

Electronic configuration of actinium and 5f-series of f-block elements:

i. The 5f-series includes elements from thorium (Th) to lawrencium (Lr).

The electronic configuration of these elements can be expressed in terms of Rn (Z = 86).

ii. Electronic configuration of Rn (Z = 86) = 1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰4s² 4p⁶ 4d¹⁰5s² 5p⁶ 5d¹⁰5f¹⁴6s² 6p⁶ .

Therefore, General electronic configuration of actinoids : [Rn] 5f^1-14 6d^0-1 7s²

iii. The electronic configuration of actinium (Z = 89) is [Rn] 5f⁰ 6d¹ 7s² . It does not contain any 5f electron.

iv. Thorium (Z = 90) has expected electronic configuration [Rn] 5f¹ 6d¹ 7s² and observed electronic configuration [Rn] 5f⁰ 6d² 7s² . This can be explained on the basis of extra stability of empty 5f orbital. 

The electrons are successively added to 5f subshell.

Element	Symbol	Atomic Number	Electronic configuration (Expected)	Electronic configuration (Observed)
Actinium	Ac	89	[Rn] 5f0 6d1 7s2	[Rn] 5f0 6d1 7s2
Thorium	Th	90	[Rn] 5f1 6d1 7s2	[Rn] 5f0 6d2 7s2
Protactinium	Pa	91	[Rn] 5f2 6d1 7s2	[Rn] 5f2 6d1 7s2
Uranium	U	92	[Rn] 5f3 6d1 7s2	[Rn] 5f3 6d1 7s2
Neptunium	Np	93	[Rn] 5f4 6d1 7s2	[Rn] 5f4 6d1 7s2
Plutonium	Pu	94	[Rn] 5f5 6d1 7s2	[Rn] 5f6 6d0 7s2
Americium	Am	95	[Rn] 5f7 6d0 7s2	[Rn] 5f7 6d0 7s2
Curium	Cm	96	[Rn] 5f7 6d1 7s2	[Rn] 5f7 6d1 7s2
Berkelium	Bk	97	[Rn] 5f8 6d1 7s2	[Rn] 5f9 6d0 7s2
Californium	Cf	98	[Rn] 5f9 6d1 7s2	[Rn] 5f10 6d0 7s2
Einsteinium	Es	99	[Rn] 5f10 6d1 7s2	[Rn] 5f11 6d0 7s2
Fermium	Fm	100	[Rn] 5f11 6d1 7s2	[Rn] 5f12 6d0 7s2
Mendelevium	Md	101	[Rn] 5f12 6d1 7s2	[Rn] 5f13 6d0 7s2
Nobelium	No	102	[Rn] 5f14 6d0 7s2	[Rn] 5f14 6d0 7s2
Lawrencium	Lr	103	[Rn] 5f14 6d1 7s2	[Rn] 5f14 6d1 7s2