The Lanthanoids

Step 1: Identifying the first element of the rare earth series.
The rare earth elements include the lanthanides and actinides. The first element of the lanthanide series is Cerium.

Step 2: Explanation of the options.
- (1) Cerium: Cerium is the first element in the lanthanide series of rare earth elements.
- (2) Cesium: Cesium is an alkali metal, not a rare earth metal.
- (3) Lanthanide: Lanthanide refers to the series of elements and not a specific element.
- (4) Actinide: Actinide is a series of elements, but cerium is part of the lanthanide series, not the actinide series.

Step 3: Conclusion.
The correct answer is (1) Cerium.

Step 1: Formula / Definition}

Step 2: Calculation / Simplification}
(a) Contains keto group forms oxime with .
(b) Keto-enol tautomerism due to acidic -hydrogens.
(c) group positive iodoform test.
All statements are true.
Step 3: Final Answer

Concept: Lanthanide contraction is the gradual decrease in atomic size across lanthanide series due to poor shielding of f-electrons.

Step 1: Effect on elements
Zr (4d) and Hf (5d) have nearly same atomic size due to contraction.

Step 2: Reason
Extra electrons in f-orbitals do not shield effectively → nucleus pulls electrons closer. Conclusion:
Thus, Zr and Hf have almost equal size.

Concept:
•Colour in ions arises due to or transitions
•Half-filled and fully-filled orbitals show very weak or no transitions

Step 1: Electronic configurations:
• (no unpaired electrons)
•
• (half-filled)
• (fully-filled)

Step 2:
• has 7 unpaired electrons (half-filled stable)
•No effective transition colourless Final: