The formation of the oxide ion, ${\mathrm{O}}^{2-} \left(\mathrm{g}\right)$, from oxygen atom requires first an exothermic and then an endothermic step as shown below:

$\mathrm{O} \left(\mathrm{g}\right) + {\mathrm{e}}^{-}\to {\mathrm{O}}^{-} \left(\mathrm{g}\right) ; △ {\mathrm{H}}^{⊝}=-141 \mathrm{KJ} {\mathrm{mol}}^{-1}$

${\mathrm{O}}^{-} \left(\mathrm{g}\right) + {\mathrm{e}}^{-}\to {\mathrm{O}}^{2-} \left(\mathrm{g}\right) ; △ {\mathrm{H}}^{⊝}=+780 \mathrm{KJ} {\mathrm{mol}}^{-1}$

Thus, the process of formation of ${\mathrm{O}}^{2-}$ In gas-phase Is unfavourable even though ${\mathrm{O}}^{2-}$ Is isoelectronic with neon. It Is due to the fact that, 

Comprehension given below is followed by a question. Choose the correct option.
In the modern periodic table, elements are arranged In order of increasing atomic numbers which is related to the electronic configuration. Depending upon the type of orbitals receiving the last electron, the elements in the periodic table have been divided into four blocks, viz, $\mathrm{s}, \mathrm{p}, \mathrm{d} \mathrm{and} \mathrm{f}$. The modern periodic table consists of $7$ periods and $18$ groups. Each period begins with the filling of a new energy shell. In accordance with the Aufbau principle, the seven periods $(1 \mathrm{to} 7)$ have $2, 8, 8, 18, 18, 32$ and $32$ elements respectively. To avoid the periodic table being too long, the two series of $\mathrm{f}$-block elements, called lanthanoids and actinoids are placed at the bottom of the main body of the periodic table.

The element with atomic number $57$ belongs to 

The last element of the $\mathrm{p}$-block in $6^{\mathrm{th}}$ period is represented by the outermost electronic configuration. 

Comprehension given below is followed by a question. Choose the correct option.
In the modern periodic table, elements are arranged In order of increasing atomic numbers which is related to the electronic configuration. Depending upon the type of orbitals receiving the last electron, the elements in the periodic table have been divided into four blocks, viz, $\mathrm{s}, \mathrm{p}, \mathrm{d} \mathrm{and} \mathrm{f}$. The modem periodic table consists of $7$ periods and $18$ groups. Each period begins with the filling of a new energy shell. In accordance with the Aufbau principle, the seven periods $(1 \mathrm{to} 7)$ have $2, 8, 8, 18, 18, 32$ and $32$ elements respectively. To avoid the periodic table being too long, the two series of $\mathrm{f}$-block elements, called lanthanoids and actinoids are placed at the bottom of the main body of the periodic table.

Which of the elements whose atomic numbers are given below, cannot be accommodated in the present set up of the long form of the periodic table?  

Comprehension given below is followed by a question. Choose the correct option.
In the modern periodic table, elements are arranged In order of increasing atomic numbers which is related to the electronic configuration. Depending upon the type of orbitals receiving the last electron, the elements in the periodic table have been divided into four blocks, viz, $\mathrm{s}, \mathrm{p}, \mathrm{d} \mathrm{and} \text{f}$. The modem periodic table consists of $7$ periods and $18$ groups. Each period begins with the filling of a new energy shell. In accordance with the Aufbau principle, the seven periods $(1 \mathrm{to} 7)$ have $2, 8, 8, 18, 18, 32$ and $32$ elements respectively. To avoid the periodic table being too long, the two series of $\mathrm{f}$-block elements, called lanthanoids and actinoids are placed at the bottom of the main body of the periodic table.

 The electronic configuration of the element which is just above the element with the atomic number $43$ in the same group is _____. 

Comprehension given below is followed by a question. Choose the correct option.
In the modern periodic table, elements are arranged In order of increasing atomic numbers which is related to the electronic configuration. Depending upon the type of orbitals receiving the last electron, the elements in the periodic table have been divided into four blocks, viz. $\mathrm{s}, \mathrm{p}, \mathrm{d} \mathrm{and} \text{f}$. The modem periodic table consists of $7$ periods and $18$ groups. Each period begins with the filling of a new energy shell. In accordance with the Aufbau principle, the seven periods $(1 \mathrm{to} 7)$ have $2, 8, 8, 18, 18, 32$ and $32$ elements respectively. To avoid the periodic table being too long, the two series of $\mathrm{f}$-block elements, called lanthanoids and actinoids are placed at the bottom of the main body of the periodic table.

The elements with atomic numbers $35,53 \mathrm{and} 85$ are all __________.

Electronic configurations of four elements $\mathrm{A}, \mathrm{B}, \mathrm{C} \mathrm{and} \mathrm{D}$ are given below

(A) $1{\mathrm{s}}^{2}2{\mathrm{s}}^{2}2{\mathrm{p}}^6$

(B) $1{\mathrm{s}}^{2}2{\mathrm{s}}^{2}2{\mathrm{p}}^4$

(C) $1{\mathrm{s}}^{2}2{\mathrm{s}}^{2}2{\mathrm{p}}^{6}3{\mathrm{s}}^1$

(D) $1{\mathrm{s}}^{2}2{\mathrm{s}}^{2}2{\mathrm{p}}^5$

Which of the following is the correct order of increasing tendency to gain electron?