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In the modern periodic table, elements a...

In the modern periodic table, elements are arranged in order of increasing atomic numbers, which is related to the electornic configuration. Depending upon the type of orbitals receiving the last electron, the elements in the periodic table have been divided into four blocks, viz, 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 principal, 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

A

s-block

B

p-block

C

d-block

D

f-block

Text Solution

Verified by Experts

The correct Answer is:
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 (Z = 56) , the addition of the next electron (i.e., 57th) should occur in 4f-orbital in accordance aufbau principle. This will however, tend to destblize the xenon core (Z = 54), [Kr](`4d^(10) 4f^(0) 5s^(2) 5p^(6) 5d^(0)`) since the 4f-orbital 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 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 4f-orbital.
Thus, the outer electronic configuration of La(Z = 57) is `5d^(1) 6s^(2)` rather than the expected `4f^(1) 6s^(2)`.
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