Ncert Exercises & Solutions

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 configuration. Depending upon the type of orbitals receiving the last electron, the elements in the periodic table have been divided into four block, 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 actinoids are placed at the bottom of the periodic table

(i) The element with atomic number 57 belongs to

1. s-block

2. p-block

3. d-block

4. f-block

(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 with Aufbau principle. This will however, tend to destabilize the xenon core (Z=54), [Kr]

(4 d^{10} 4 f^{0} 5 s^{2} 5 p^{6} 5 d^{0})

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 that of 4f-orbital. In doing so, the stability 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

5 d^{1} 6 s^{2}

rather than the expected

4 f^{1} 6 s^{2} .

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