Which of the following is the correct order of size of the given species:

The period number in the long form of the periodic table is equal to

the elements in which electrons are progressively filled in 4f-orbital are called

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,

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