The f-block elements are those in which the differentiating electron enters the (n-2)f orbital. There are two series of f-block elements corresponding to filling of 4f and 5f-orbitals called lanthanides and actinides respectively. They show different oxidation states depending upon the stability of `f^0, f^7`and `f^14` configurations, though the principal oxidation state is +3. There is a regular decrease in size of lanthanide ions with increase in atomic number and it is known as lanthanide contraction. As a result of this, the basic character of oxides and hydroxides decreases from first element (La) to last element (Lu). All the actinides are radioactive and therefore, it is difficult to study their chemical nature.
The atomic numbers of three lanthanide elements X,Y and Z are 65,68 and 70 respectively. The basic character of their hydroxides will decrease as

Options

A. X gt Y gt Z
B. X gt Z gt Y
C. Z gt Y gt X
D. Z gt Y gtX

Explanation

To solve the problem regarding the basic character of the hydroxides of the lanthanide elements with atomic numbers 65 (X), 68 (Y), and 70 (Z), we will follow these steps:

### Step 1: Identify the Elements
The atomic numbers correspond to the following lanthanide elements:
- X (65): Terbium (Tb)
- Y (68): Erbium (Er)
- Z (70): Ytterbium (Yb)

### Step 2: Understand the Concept of Lanthanide Contraction
The lanthanide contraction refers to the gradual decrease in the size of lanthanide ions as the atomic number increases. This occurs due to the poor shielding effect of the f-electrons, leading to a greater effective nuclear charge felt by the outer electrons.

### Step 3: Analyze the Basic Character of Hydroxides
The basic character of hydroxides generally decreases with increasing covalent character. As the size of the lanthanide ions decreases (due to lanthanide contraction), the covalent character of their hydroxides increases.

### Step 4: Order the Basic Character
Since the basic character decreases with increasing atomic number due to the decrease in ionic size and increase in covalent character, we can establish the following order:
- X (Tb, 65) will have the highest basic character.
- Y (Er, 68) will have a lower basic character than X.
- Z (Yb, 70) will have the lowest basic character.

### Conclusion
Thus, the order of basic character of the hydroxides of elements X, Y, and Z is:
**Basic Character: Tb (X) > Er (Y) > Yb (Z)**