Question

Which of the following is the most basic?
A.\[{\text{Ce}}{\left( {{\text{OH}}} \right)_3}\]
B.\[{\text{Lu}}{\left( {{\text{OH}}} \right)_3}\]
C.\[{\text{Yb}}{\left( {{\text{OH}}} \right)_3}\]
D.\[{\text{Al}}{\left( {{\text{OH}}} \right)_3}\]

Answer 1

Hint: As we move left to right in a lanthanide the atomic number increases and the basic strength decreases. This happens due to lanthanide contraction.

Complete step by step answer:
- Lanthanide contraction occurs due to an increase in atomic number. As the Lanthanide contraction increases the atomic size the atomic size of tri positive ions decreases. Lanthanides are also named as rare earth elements. They are the elements with atomic number 58 to 71. They belong to f block elements. F block elements are those elements in which the last electron goes to the anti penultimate shell.
- The gradual decrease in atomic size occurs as the atomic number increases and this phenomenon is known as lanthanide contraction. This lanthanide contraction brings change in atomic size, difficulty in separation of lanthanides, effect on the basic strengths of lanthanides, their tendency to form complexes and the ionization energy of d block elements.
- Basic character decreases from cerium to lutetium. This is because of an increase in lanthanide contraction due to an increase in atomic number. The radius will decrease and hence the attraction for the outermost electron increases which leads to lesser donation of hydroxide and hence less basic will be. Lesser will be the atomic size, more will be the tendency of the nucleus to attract the electrons and hence tougher will be the release of hydroxide. A strong base is that which leaves hydroxide easily.

The most basic among the given options is option A.

Note:
The f block elements are also known as inner transition elements. This is because their atomic number lies in between the transition metal elements however they are placed at the bottom of the modern periodic table.