Describe the cause of the following variations with respect to lanthanoids and actinoids:
(i) Greater range of oxidation states of actinoids as compared to lanthanoids.
(ii) Greater actinoid contraction as compared to lanthanoid contraction.
(iii) Lower ionisation enthalpy of early actinoids as compared to the early lanthanoids.
Answers
Lanthanides and actinides
Explanation:
Lanthanides and actinides are elements with unfilled f orbitals. Lanthanides are all metals with reactivity similar to group 2 elements. Actinides are all radioactive elements.
Actinides exhibits larger oxidation states than lanthanides, because of very small energy gap between 5f, 6d and 7s sub shells. Thus, the outermost electrons get easily excited to the higher energy levels, giving variable oxidation states.
The main reason of the actinide contraction is the imperfect shielding of the one 5f-electron by another 5f- electron in the same shell. Due to poor shielding effect of the 5f electrons, effective nuclear charge increases which causes the contraction in atomic size.
Actinoids have low ionisation enthalpy than lanthanoids because 5f electrons will have more effective shielding than 4f electrons. The outer electrons are not tightly held and hence responsible for bonding.