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. 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, 
and 
.
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 
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
(i) The element with atomic number 57 belongs to:
Text Solution
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(i) , (ii) , (iii)
(i) ( c) 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 greater stability of the xenon (inert gas) core.
After barium 
, the addition of the next electron (i.e., 
) should occur in 4 f -orbital in accordance with Aufbau principle. This will however, tend to destabilize the xenon core 
since the 4f -orbitals lie inside the core.
Therefore, the 
electron prefers to enter 5d -orbital which lies outside the xenon core and whose energy is only slightly higher than that 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 4 f - orbital.
Thus, the outer electronic configuration of 
is 
rather than the expected 
.
(ii) The last element of p-block in 6th period is 
. It's outer electronic configuration is 
.
(iii) The long form of the periodic table contain element with atomic number 1 to 118.
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