Question

The electronic configuration of ${\text{Eu}}$ (atomic number $63$ ), ${\text{Gd}}$ (atomic number $64$ ) and ${\text{Tb}}$ (atomic number $65$ ) are:
A. $\left[ {{\text{Xe}}} \right]4{{\text{f}}^7}6{{\text{s}}^2},\left[ {{\text{Xe}}} \right]4{{\text{f}}^8}6{{\text{s}}^2}$ and $\left[ {{\text{Xe}}} \right]4{{\text{f}}^8}5{{\text{d}}^1}6{{\text{s}}^2}$
B. $\left[ {{\text{Xe}}} \right]4{{\text{f}}^7}5{{\text{d}}^1}6{{\text{s}}^2},\left[ {{\text{Xe}}} \right]4{{\text{f}}^7}5{{\text{d}}^1}6{{\text{s}}^2}$ and $\left[ {{\text{Xe}}} \right]4{{\text{f}}^9}6{{\text{s}}^2}$
C. $\left[ {{\text{Xe}}} \right]4{{\text{f}}^6}5{{\text{d}}^1}6{{\text{s}}^2},\left[ {{\text{Xe}}} \right]4{{\text{f}}^7}5{{\text{d}}^1}6{{\text{s}}^2}$ and $\left[ {{\text{Xe}}} \right]4{{\text{f}}^8}5{{\text{d}}^1}6{{\text{s}}^2}$
D. $\left[ {{\text{Xe}}} \right]4{{\text{f}}^7}6{{\text{s}}^2},\left[ {{\text{Xe}}} \right]4{{\text{f}}^7}5{{\text{d}}^1}6{{\text{s}}^2}$ and $\left[ {{\text{Xe}}} \right]4{{\text{f}}^9}6{{\text{s}}^2}$

Answer 1

Electronic configuration is the arrangement of electrons in the orbital of an atom. We must use the atomic number for the number of electrons and the periodic table for the order of filling electron orbitals. Condensed configurations consist of preceding noble gas and outer electrons.

Complete step by step answer:
Electronic configuration is described by a number that designates the number of principal shells, a letter that designates the subshell, a subscript that designates the number of electrons in that particular subshell or orbital. The energy levels are designated by ${\text{s}},{\text{p}},{\text{d}},{\text{f}}$ etc.
Aufbau principle tells that electrons in an atom occupy first the lowest possible energy levels and orbitals. The order is $1{\text{s,}}2{\text{s,}}2{\text{p,}}3{\text{s,}}3{\text{p,}}4{\text{s,}}3{\text{d,}}4{\text{p,}}5{\text{s,}}4{\text{d,}}5{\text{p,}}6{\text{s,}}4{\text{f,}}5{\text{d,}}6{\text{p,}}7{\text{s,}}5{\text{f,}}6{\text{d,}}7{\text{p}}$
Pauli’s exclusion principle defines that the spins have to be paired and only two electrons can be filled.
Hund’s rule states that when you get to orbitals having the same energy, you fill them all half way first, and then you start pairing up the electrons.
Moreover, the maximum number of electrons allowed per energy level is denoted by $2{{\text{n}}^2}$, where ${\text{n}}$ is the principal energy level.
The given options represent the condensed configuration. The given elements have atomic numbers $63,64,65$ respectively. The elements which start atomic numbers from $58 - 71$ comes under the lanthanide group.
To represent in condensed configuration, we need to know their nearest noble gas. So Xenon is the nearest noble gas. We can represent the electronic configuration with respect to Xenon. Xenon has atomic number $54$. Xenon has an outermost shell $5{{\text{s}}^2}5{{\text{p}}^6}$. So we have to represent only the remaining.
i.e. For ${\text{Eu}}$, it has remaining nine electrons. So it is represented as $\left[ {{\text{Xe}}} \right]4{{\text{f}}^7}6{{\text{s}}^2}$.
For ${\text{Gd}}$, it has remaining ten electrons and it has a single electron filled in ${\text{d}}$ orbital. Thus it is represented as $\left[ {{\text{Xe}}} \right]4{{\text{f}}^7}5{{\text{d}}^1}6{{\text{s}}^2}$
For ${\text{Tb}}$, it has remaining eleven electrons and it has empty ${\text{d}}$ orbital. Therefore its electronic configuration is $\left[ {{\text{Xe}}} \right]4{{\text{f}}^9}6{{\text{s}}^2}$.

So, the correct answer is Option D .

Note:
The three rules that govern electron configurations are Aufbau principle, Pauli exclusion principle and Hund’s rule. Generally, heavier elements are represented in condensed configuration since it is difficult to represent it in long. The atomic number gives us a short idea of where the element is placed.