8/3/2019 Elements Electric Configuration Assignment Experts Mind

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Chemistry Assignment | ExpertsMind.com

Elements Electric Configuration

In the transition elements, d-orbitals of penultimate shell are progressively filled. There are mainly three

transition series. The first transition series involves the fitting of 3-d orbitals. It starts from scandium (Z = 21) and

goes up to zinc (Z = 30).

The second transition series involves the fitting of 4d- orbitals and includes 10 elements from yttrium (Z = 39) to

cadmium (Z = 48).

The third transition series involves filling of 5d- orbitals. The first element of this series is lanthanum (Z = 57). It is

followed by fourteen elements called lanthanoids which involves the filling of 4f- orbitals. The next nine element

from hafnium (Z = 72) to mercury (Z = 80) belong to third transition series.

The fourth transition series which begins with actinium is still incomplete.

The general electronic configuration of transition elements is (n 1)d1-10ns1-2.

It may be noted that the electronic configurations of zinc, cadmium and mercury can be represented by the

general formula (n 1)d10 ns2. The d-subshell in these elements in completely filled in the ground state as well a

in their common oxidation states. Because of this reason, the elements Zn, Cd, and Hg are not regarded as

transition elements.

It would be of interest to consider the change in the relative energies of various subshells with change in atomic

number. As is clear upto Z = 20, 4s subshell has lower energy than 3d subshell. Therefore, calcium(Z = 20) has the

electronic configuration 1s2 2p2 2p6 3s2 3p6 4s2. Beyond this there is a sharp decrease in the energy of 3d-

subshell and its energy becomes less than the 4s-subshell. Starting with scandium (Z = 21) up to zinc (Z = 30), the

3d-orbitals are filled and the d-electrons become more effective in shielding the 4s electrons from the attractiveforce of the nucleus. Therefore, 3d-subshell gets pulled lower than 4s. It may be noted that when atoms of these

elements change into cations, the electrons are removed from 4s-orbitals instead of 3d-orbitals although the

former was filled earlier. For example,

26Fe : [Ar] 3d6 4s2 Fe2+ : [Ar] 3d6

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27Co : [Ar] 3d7 4s2 Co2+ : [Ar] 3d7

Similar pattern is again repeated in the fifth period where the 5s, 4d-and 6s-, 5d- orbitals are involved in second

and third transition series respectively. In lanthanum, however, the energies of 4f-, 5d- and 6s- orbitals are very

close to one another and one electron enters the 5d- orbital before 4f- begins to fill.

The outer electronic configuration of lantahnium is 5d1 6s2. In the subsequent elements, the 4f- subshell is

stabilized relative to the 5d- subshell. Since there are seven 4f- orbitals, a total number of 14 elements result

before the 5d- orbitals begin to fill up. The transition elements consist of the elements scandium (Z = 21) to zinc

= 30); yttrium (Z = 89), which involve the filling of their 3d- 4d-, 5d- and 6d- orbitals.

The fourteen elements immediately following lantahnium constitute the first inner transition series. These

elements are from cerium (Z = 58) to lutetium (Z = 71) in which the 4f- orbitals are being filled progressively.

These are called lanthanoids. The elements of second inner transition series are called actinoids. They consist of

fourteen elements from thorium (Z = 90) to lawrencium (Z = 103). In these elements, the 5d-orbitals are

successively filled.