Electronic Configurations of D Block Elements

Groups 3 to 12 elements are called d-block or transition elements. These elements are present between p-block and s-block elements in the periodic table. These elements’ properties are intermediate between the properties of s -block and p -block elements, i.e. d -block elements represent a change or transition in properties from most electropositive s - block elements to less electropositive p - block elements. Therefore, these elements are called transition elements.

The d-block elements include the most common metals used in construction and manufacturing, metals that are valued for their beauty (gold, silver and platinum), metals used in coins (nickel, copper) and metals used in modern technology (titanium). 

In the transition element, the last differentiating electron is accommodated on penultimate d-orbitals, i.e., d-orbitals are successively filled. The general electronic configuration of transition elements is:

(n-1)^1-10 ns ^{0,1 or 2}

There are four complete rows (called series) of ten elements each corresponding to filling of 3d, 4d, 5d and 6d-orbitals respectively. Each series starts with a member of group third (IIIB) and ends with a member of group twelve (IIB).

Why are D-block elements also referred to as Transition elements (In Brief)?

Groups 4-11 are made up of transition components. Transition elements include scandium and yttrium from Group 3, which have a partly filled d subshell in the metallic form. Elements in the 12 columns of the d block, such as Zn, Cd, and Hg, have entirely filled d-orbitals and are hence not considered transition elements.

Transition Elements get their name from the fact that they are placed between s and p block elements and have characteristics that transition between them. So, while all transition metals are d block elements, they are not all transition elements. Filling Transition Metal Orbitals

The first-row transition metal electron configuration consists of 4s and 3d subshells with a core of argon (noble gas). This applies only to transition metals in the first row, adjustments are required when writing the electron configuration for the other transition metal rows. Before the first row of transition metals, the noble gas would be the core written around the element symbol with brackets (i.e. Ar-Ar would be used for the first row of transition metals), and the electron configuration would follow an Ar-Ar nsxndx format. The electron configuration for first-row transition metals would simply be Ar-Ar 4sx3dx. Based on the periodic table, the energy level, "n," can be determined simply by looking at the row number in which the element is located. There is, however, an exception for the d-block and f-block, where the energy level, "n" for the d-block is "n-1" ("n" minus 1) and "n-2" for the f-block (see the following periodic classification table). The "x" in nsx and ndx, in this case, is the number of electrons in a particular orbital (i.e. s - orbitals can hold up to 2 electrons, p - orbitals can hold up to 6 electrons, d - orbitals can hold up to 10 electrons, and f - orbitals can hold up to 14 electrons). To determine what "x" is, simply count the number of boxes you will find before you reach the element you are trying to determine the configuration of the electron.

First transition or 3d-series: 

Elements: Sc(21) to Zn(30). 3d-orbitals are gradually filled up.

The actual configurations are explained on the basis of the stability concept of half-filled or completely filled (n-l) d-orbitals. (n-l) d-subshell is more stable when 5 or 10 electrons are present, i.e., every d-orbital is either singly occupied or doubly occupied.

Second Transition or 4d-series

This series consists of elements from Y(39) to Cd(48). 4d-orbitals are gradually filled up.

Elements marked with an asterisk have anomalous configurations. Nuclear-electron and electron-electron forces are attributed factors.

Third Transition or 5d-series: 

This series consists of elements from La(S7) to Hg(80) except 14 elements of lanthanide series from Ce(S8) to Lu(71). 5d-orbitals are gradually filled up.

Fourth Transition or 6d-series 

This series consists of elements from Ac(89) to Uub(112) except 14 elements of the actinide series from Th(90) to Lr(103). 6d-orbitals are gradually filled up.

Variable Oxidation State of D-block Elements 

The oxidation state is a notional condition in which the atom seems to lose or gain more electrons than it does in its normal valency state. It's still useful for understanding the atom's characteristics.  Both s and d-orbitals can have electrons in transition elements.

Because the energy difference between the s and d orbitals is modest, both electrons can participate in the production of ionic and covalent bonds, resulting in multiple(variable) valency states (oxidation states).

As a result, any transition element can have a minimum oxidation state equal to the number of s-electrons and a maximum oxidation state equal to the total number of electrons in both s and d-orbitals. Between oxidation states, new oxidation states become feasible.

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