Electronic configuration

The 3d series begins with Sc (Z = 21) and ends with Zn (Z = 30). Argon, Ar, is the noble gas preceding the 3d series, and its electronic configuration is $\mathrm{1s^2\,2s^2\,2p^6\,3s^2\,3p^6}$. Calcium (Z = 20), belonging to the 's' block of the 4th period, has the electronic configuration $\mathrm{1s^2\,2s^2\,2p^6\,3s^2\,3p^6\,4s^2}$. Hence the 21st electron in scandium (Z = 21) enters the available 3d orbital. The electronic configuration of Sc is written as $\mathrm{1s^2\,2s^2\,2p^6\,3s^2\,3p^6\,3d^1\,4s^2}$, or can also be represented as $\mathrm{[Ar]\,3d^1\,4s^2}$.

The electronic configuration of the elements of the 3d series is given in Table 8.2. Since zinc has a completely filled (n-1)d orbital in the ground state ($\mathrm{3d^{10},4s^2}$) and ($\mathrm{3d^{10}}$) in its common oxidation state +2, it is not regarded as a transition element. On the same ground, cadmium and mercury from the 4d and 5d series are not considered as transition elements. Copper in the elementary state ($\mathrm{3d^{10}\,4s^1}$) contains filled 3d orbitals, but in the +2 oxidation state it has a partly filled 3d orbital ($\mathrm{3d^9}$) — hence copper is a transition element.

The general electronic configuration of the four series of d-block elements of the periodic table can be represented as given below:

• i. 3d series : $\mathrm{[Ar]\ 3d^{1-10}\ 4s^2}$
• ii. 4d series : $\mathrm{[Kr]\ 4d^{1-10}\ 5s^{0-2}}$
• iii. 5d series : $\mathrm{[Xe]\ 4f^{14}}$(except La) $\mathrm{5d^{1-10}\ 6s^2}$
• iv. 6d series : $\mathrm{[Rn]\ 5f^{14}}$(except Ac) $\mathrm{6d^{1-10}\ 7s^2}$

Table 8.2 Electronic configuration of 3d series of d-block elements