Which of the following are d-block elements but not regarded as trans…

The d-block elements, also known as transition elements, are a group of elements located in the middle of the periodic table. They include elements from group 3 (Scandium) to group 12 (Zinc), and they are characterized by the presence of partially filled d orbitals in their electronic configurations.

The d orbitals are part of the second innermost electron shell, and they can accommodate a maximum of 10 electrons. In transition elements, one or more of these d orbitals are partially filled in their neutral or common oxidation states. This partially filled d orbital allows these elements to exhibit unique properties, such as multiple oxidation states, colorful compounds, and the ability to form complex ions.

Let's look at the electronic configurations of the elements mentioned in the options:

1. Copper (Cu) - [Ar] 3d¹⁰ 4s¹
   Silver (Ag) - [Kr] 4d¹⁰ 5s¹
   Gold (Au) - [Xe] 4f¹⁴ 5d¹⁰ 6s¹

Copper, silver, and gold have partially filled d orbitals in their common oxidation states, making them transition elements. They show various oxidation states, form colorful complexes, and exhibit other typical transition metal properties.

2. Iron (Fe) - [Ar] 3d⁶ 4s²
   Cobalt (Co) - [Ar] 3d⁷ 4s²
   Nickel (Ni) - [Ar] 3d⁸ 4s²

Iron, cobalt, and nickel are also transition elements as they have partially filled d orbitals in their common oxidation states, allowing them to display a variety of oxidation states and other transition metal characteristics.

3. Zinc (Zn) - [Ar] 3d¹⁰ 4s²
   Cadmium (Cd) - [Kr] 4d¹⁰ 5s²
   Mercury (Hg) - [Xe] 4f¹⁴ 5d¹⁰ 6s²

Zinc, cadmium, and mercury have completely filled d orbitals in their neutral or common oxidation states. Zinc and cadmium always have a +2 oxidation state, while mercury typically exhibits +1 and +2 oxidation states. Since their d orbitals are completely filled, they do not show the characteristic properties of transition elements, and thus, they are not considered transition elements.

Instead, zinc, cadmium, and mercury are classified as d-block elements or "post-transition metals." They still possess some properties typical of metals, but they lack the diverse oxidation states and colorful compounds observed in transition elements due to their filled d orbitals.

In summary, while zinc, cadmium, and mercury are d-block elements, they are not regarded as transition elements because their d orbitals are completely filled in their common oxidation states, preventing them from showing the typical behavior of transition elements.