Q11 of 59 Page 27

The formation of the oxide ion, O2– (g), from oxygen atom requires first an exothermic and then an endothermic step as shown below:

O (g) + e– → O– (g) ; ∆ Hς = – 141 kJ mol–1


O– (g) + e– → O2– (g); ∆ Hς = + 780 kJ mol–1


Thus process of formation of O2– in gas phase is unfavourable even though O2– is isoelectronic with neon. It is due to the fact that,


Ois an electronegative element so when we add an electron to it, it gains that electron and forms O- anion. Thus energy is released in the formation and it is an exothermic process. But while another electron is added to this unstable O- anion then it faces repulsion due to the similar charges. So in order to overcome these repulsions due to similar charge external energy is required due to which this becomes an endothermic process.

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Comprehension given below is followed by some multiple choice questions. Each question has one correct option.

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, s, p, d and 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 Arfbau principle, the seven periods (1 to 7) have 2, 8, 8, 18, 18, 32 and 32 elements respectively. The seventh period is still incomplete. To avoid the periodic table being too long, the two series of 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


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Comprehension given below is followed by some multiple choice questions. Each question has one correct option.

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, s, p, d and 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 Arfbau principle, the seven periods (1 to 7) have 2, 8, 8, 18, 18, 32 and 32 elements respectively. The seventh period is still incomplete. To avoid the periodic table being too long, the two series of f-block elements, called lanthanoids and actinoids are placed at the bottom of the main body of the periodic table.


The last element of the p-block in 6th period is represented by the outermost electronic configuration.