Question

The lanthanide series starts from and ends in which atomic number?
A. $55,68$
B. $56,69$
C. $57,70$
D. $58,71$

Answer 1

Hint: The lanthanide series are a group of elements which are similar in characteristics to the element Lanthanum. This series starts with the element Cerium and ends with Lutetium. Thus, by finding out their atomic numbers by looking at the periodic table, we can determine the range of the lanthanide series.

Complete step by step answer:
Lanthanides are a series of fourteen elements, which extend from Cerium to Lutetium. They are part of the f-block elements, since valence electrons get filled into f orbitals in these elements. Let us see why they are called so:
The name Lanthanoids means “lanthanum-like”, since these elements possess characteristics similar to that of Lanthanum. Looking at the periodic table, we see that these are formed as a hidden series in the sixth period, sandwiched between the third and fourth groups. Looking at the atomic numbers at both ends, we see that the first element Cerium has an atomic number of $58$ while the last one, Lutetium has an atomic number of $71$.

So, the correct answer is Option D .

Additional information:
All the lanthanides are known to exhibit a common oxidation state of $ + 3$. A common phenomenon observed with the lanthanides is the lanthanoid contraction, which is the greater-than-expected reduction in atomic radius caused by the poor shielding of the $4f$ electrons.

Note: Lanthanum is not considered to be part of the lanthanide series, since it does not have any electrons in its f orbital. Below the lanthanide series is the actinide series, in which the electrons get filled into the $5f$ orbital. Together the lanthanide and actinide series make up the f-block elements. Note that many of the f-block elements are radioactive, and most of them are rarely found on earth.