Question

State the position of cerium (Z = 58) and uranium (Z = 92) in periodic table.

Answer 1

Hint: To know the position of elements we must know its group number and period number. Group number is calculated using the valence shell electrons and period number is the number of shells contained in an element.

Complete step by step answer:
To know the position of any element we must know the vertical and horizontal component in a periodic table. The vertical component of a periodic table is group and horizontal component is period. To calculate the period and group, we must write the electronic configuration. We will first get to know the block number and then the group number.
-The electronic configuration of cerium is:
\[[{\text{Xe}}]4{{\text{f}}^1}5{{\text{d}}^1}6{{\text{s}}^2}\]
The last electron is going in an anti-penultimate f orbital and hence it is an f block element. As we see it contains 6 shells 1, 2, 3, 4, 5 and 6. Hence the period number is 6. In f block elements group number is not defined.
-The electronic configuration of uranium is:
\[[{\text{Rn}}]5{{\text{f}}^3}{\text{6}}{{\text{d}}^1}{\text{7}}{{\text{s}}^2}\]
The last electron is going in an anti-penultimate f orbital and hence it is also an f block element. As we see it contains 7 shells 1, 2, 3, 4, 5, 6 and 7. Hence the period number is 7. In f block elements group number is not defined.

Note:
Since the higher atomic number makes the electronic configuration very long. So instead of writing the whole configuration we write the preceding Noble gas configuration, the noble gas before cerium is xenon and the noble gas before uranium is radon. Hence we write it as electronic configuration.