(i) 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 Aufbau 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 actionoids are placed at the bottom of the main body of hte periodic table
The element with atomic number 57 belongs to
(a) s- block ( b) p-block (c) d- block (d) f- block

(ii) The last element of the p - block in 6th period is represented by the outermost electronic configuration.
(a) $7s^{2}7p^6$ (b) $5f^{14}6d^{10}7s^{2}7p^0$
(c) $4f^{14}5d^{10}6s^{2}6p^6$ (d) $4f^{14}5d^{10}6s^{2}6p^4$

(iii) Which of the elements whose atomic numbers are given below, cannot be accommodated in the present set up of the long frm of the periodic table?
(a) 107 (b) 118 (c) 126 (d) 102

(iv) The electronic configuration of the element which is just above the element with atomic number 43 in the same group is .....
(a) $1s^{2}2s^{2}2p^{6}3s^{2}3p^{6}3d^{5}4s^2$
(b) $1s^{2}2s^{2}2p^{6}3s^{2}3p^{6}3d^{5}4s^{3}4p^6$
(c) $1s^{2}2s^{2}2p^{6}3s^{2}3p^{6}3d^{6}4s^2$
(d) $1s^{2}2s^{2}2p^{6}3s^{2}3p^{6}3d^{7}4s^2$

(v) The elements with atomic numbers 35, 53 and 85 are all ..
(a) noble gases (b) halogens (c) heavy metals (d) light metals

(i) Answer: (c) d- block

The element with atomic number 57 belongs to d- block element as the last electron enters the 5d orbital against the Aufbau principle. This anomalous behaviour can be explained on the basis of the greater stability of the xenon (inert gas) core
After barium (Z = 56) , the addition of the next electron (i.e. 57th) should occur in 4 f- orbital in accordance with Aufbau principle this will however, tend to destabilize the xenon core $(Z=54),\left[Kr\right]\left(4d^{10}4f^{2}5s^{2}5p^{6}5d^0\right)$ since the 4f-orbitals lie inside the core,
Therefore, the 57th electron prefers to enter 5d-orbital which lies outside the xenon core and whose energy is only slightly higher than of 4f- orbital. In doing so, the stability conferred on the atom due to xenon core more than compensates the slight instability caused by the addition of one electron to the higher energy 5d- orbital instead of the lower energy 4f- orbital.
Thus the outer electronic configuration of $La(Z=57)is5d^{1}6s^2$ rather than the expected $4f^{1}6s^2$

(ii) Answer: (c) $4f^{14}5d^{10}6s^{2}6p^6$

Each period starts with the filling of electrons in a new principal energy shell. Therefore 6th period starts with the filling of 6s - orbital and ends when 6 p - orbitals are completely filled,
In between 4f and 5d - orbitals are filled in accordance with Aufbau principle. Thus, the outermost electronic configuration of the last element of the p- block in the 6th period is
$6s^{2}4f^{14}5d^{10}6P^{6}or4f^{14}5d^{10}6s^{2}6p^6$

(iii) Answer: (c) 126

The long form of the periodic table contains an element with atomic number 1 to 118.

(iv) Answer: (a) $1s^{2}2s^{2}2p^{6}3s^{2}3p^{6}3d^{5}4s^2$

The fifth period begin with Rb (Z = 37) and ends at Xe (Z = 54). Thus, the element with Z = 43 lies in the 5th period. Since the 4th period has 18 elements, therefore, the atomic number of the element which lies immediately above the element with atomic number 43 is 43-18 = 25.
Now, the electronic configuration of an element with Z = 25 is
$1s^{2}2s^{2}2p^{6}3s^{2}3p^{6}3d^{5}4s^2$

(v) Answer: (b) halogens

Each period ends with a noble gas, the atomic number of noble gases (i.e., group 18 elements) are 2, 10, 18, 36, 54, and 86 therefore elements with atomic numbers 35 (36-1) 53 (54-1) and 85 (86-1) lie in a group before noble gases i.e. halogens (group 17) elements.
Thus, the elements with atomic number 35, 53 and 85 are all belongs to halogens.