The electronic configurations of \({ }_{24} \mathrm{Cr}\) and \({ }_{29} \mathrm{Cu}\) are abnormal (a) Due to extra stability of exactly half filled and exactly fully filled sub shells (b) Because they belong to d-block (c) Both the above (d) None of the above

D-block elements, often called transition metals, comprise the group of elements in the periodic table that have a partially filled d orbital at some point during their ionization process. What characterizes the d-block elements is their unique properties of being good conductors of electricity, having high melting points, and possessing the ability to form complex ions.

These elements, found in Groups 3-12, display a wide range of oxidation states and typically have a valence shell electronic configuration of (n-1)d1–10 ns1–2. The (n-1) signifies that the d orbitals are actually one energy level below the s orbital of the valence shell. This attribute contributes to the interesting chemistry of these elements, as they often engage in d orbital bonding and can form colored compounds.

Interestingly, the chemistry of d-block elements is deeply influenced by their electron configurations, which in some cases, deviate from the expected order. This deviation provides insight into the underlying stability associated with certain electron configurations found within the d-block.

The concept of electronic configuration is central to understanding the behavior of atoms in chemistry. A key factor contributing to this understanding is the stability of half-filled and fully filled subshells. These configurations are especially stable due to several reasons.

A half-filled subshell, such as that of chromium, [Ar] 3d5 4s1, benefits from symmetrical electron distribution and increased exchange energy, a form of stabilization energy that arises due to the pairing of electrons in the same subshell.

Fully filled subshells, like the 3d10 in copper's configuration, [Ar] 3d10 4s1, are stable due to the complete occupation of all the orbitals within a subshell, minimizing repulsions between electrons. These stability considerations often lead to 'exceptions' in electron configurations, where elements will adopt unusual electronic configurations to achieve this extra stability. The energetic benefit of a half-filled or fully filled d-subshell outweighs the conventional order of orbital filling.

When we encounter exceptions to the expected electron configurations in elements, it underscores the rule that atoms seek the most stable electronic arrangement. D-block elements like Chromium (Cr) and Copper (Cu) exhibit such exceptions due to the stability that comes with half-filled and fully filled d-subshells.

Chromium, with its electron configuration of [Ar] 3d5 4s1, forsakes the otherwise 'normal' filling order to achieve the added stability of a half-filled d-subshell. This is driven by the principle of maximum multiplicity, which dictates that electrons in a subshell will remain unpaired as long as possible, creating a more stable arrangement due to increased exchange energy.

In the case of Copper, the expected configuration is set aside in favor of a [Ar] 3d10 4s1 arrangement, where the complete filling of the 3d orbital provides a stability so significant that it overrides the usual filling pattern. These exceptions in electron configurations are thus not arbitrary, but favor the energetically favored state and showcase the subtleties of atomic and quantum behavior in chemistry.