Question

What is the position of elements La (Z = 57) and Ce (Z = 58) respectively in the long form of periodic table?

• A. La = Group-3, Period-6
  \hspace*{0.6cm} Ce = Group-3, Period-6
• B. La = Group-4, Period-7
  \hspace*{0.6cm} Ce = Group-5, Period-7
• C. La = Group-3, Period-7
  \hspace*{0.6cm} Ce = Group-3, Period-6
• D. La = Group-3, Period-6
  \hspace*{0.6cm} Ce = Group-3, Period-7

Hint:

Lanthanides start from La (Z = 57) and are associated with Group-3 in the long form periodic table.

Answer 1

The Correct Option is A

Step 1: Electronic configuration of La and Ce.
Lanthanum (Z = 57) has electronic configuration $[Xe]\,5d^1\,6s^2$.
Cerium (Z = 58) has electronic configuration $[Xe]\,4f^1\,5d^1\,6s^2$.

Step 2: Period determination.
Both La and Ce have their outermost electrons in the 6th shell, hence both belong to Period-6.

Step 3: Group determination.
Both elements are placed in Group-3 as they mark the beginning of the lanthanide series in the long form of the periodic table.

Step 4: Conclusion.
Therefore, the correct position is La = Group-3, Period-6 and Ce = Group-3, Period-6.