CUET UG Chemistry Booster Test – 3 Introduction & Classification

The principal quantum number (n) indicates the main energy level in which an orbital exists. For d-orbitals, the notation directly reveals the value of n. Thus, 3d orbitals belong to the third principal shell, 4d orbitals belong to the fourth shell, 5d orbitals belong to the fifth shell and 6d orbitals belong to the sixth shell. When arranging in decreasing order of principal quantum number, the orbital with the highest value of n must be placed first. Therefore, the sequence becomes 6d > 5d > 4d > 3d. These orbitals define the fourth, third, second and first transition series respectively. Understanding the relationship between orbital notation and principal quantum number is essential for classifying transition series in the periodic table.

The electronic configurations of chromium and copper are important exceptions to the expected Aufbau pattern. Half-filled and completely filled subshells possess extra stability because of symmetrical electron distribution and maximum exchange energy. Chromium achieves the stable configuration 3d⁵ 4s¹ instead of 3d⁴ 4s², while copper adopts 3d¹⁰ 4s¹ instead of 3d⁹ 4s². Therefore, Statements 1 and 3 are correct. Statement 2 is incorrect because chromium does not remain in the 3d⁴ 4s² configuration. Statement 4 is also incorrect because the energy difference between (n−1)d and ns orbitals is very small, which allows electron transfer between these orbitals. This small energy difference is responsible for many anomalous configurations observed among transition elements.

The classification of transition and inner transition elements is based on the orbital being progressively filled. The first transition series corresponds to the filling of 3d orbitals. The lanthanoid series involves progressive filling of 4f orbitals, while the actinoid series involves progressive filling of 5f orbitals. The third transition series corresponds to the filling of 5d orbitals. These classifications arise directly from the electronic configurations of the elements. The arrangement helps chemists understand periodic trends, oxidation states and chemical behaviour. Therefore, the correct matching is 3d → First Transition Series, 4f → Lanthanoids, 5f → Actinoids and 5d → Third Transition Series.

Transition and inner transition elements possess partially filled d or f orbitals that extend sufficiently towards the outer region of the atom. Because these orbitals are exposed to surrounding atoms, ligands and external fields, they strongly influence the physical and chemical properties of the elements. This leads to characteristic features such as coloured ions, variable oxidation states, magnetic behaviour and complex formation. The availability of d and f electrons allows multiple electronic transitions and different bonding possibilities. Main group elements generally lack such partially filled d or f subshells and therefore do not exhibit these properties to the same extent. Thus, the unique behaviour of transition and inner transition elements is directly related to the presence and accessibility of their partially filled d or f orbitals.

According to the IUPAC definition, a transition element is one whose atom or at least one of its common oxidation states possesses an incompletely filled d-subshell. Silver has the ground-state configuration [Kr] 4d¹⁰ 5s¹. When silver forms Ag⁺, the configuration becomes [Kr] 4d¹⁰, which still contains a completely filled d-subshell. However, in the Ag²⁺ oxidation state, one electron is removed from the 4d subshell, resulting in a 4d⁹ configuration. This incompletely filled d-subshell satisfies the IUPAC criterion for classification as a transition element. Therefore, silver is regarded as a transition metal because it can form an ion with a partially filled d-orbital.

The anomalous electronic configurations of chromium and copper arise because the energy difference between the (n−1)d and ns orbitals is very small. As a result, the transfer of one electron from the ns orbital to the d orbital requires very little energy. This transfer leads to the formation of particularly stable half-filled (d⁵) or completely filled (d¹⁰) subshells. Chromium adopts the configuration 3d⁵4s¹ instead of 3d⁴4s², while copper adopts 3d¹⁰4s¹ instead of 3d⁹4s². The extra stability is attributed to symmetrical electron distribution and increased exchange energy. These factors outweigh the small energy required for electron rearrangement. Thus, the combined effect of a small energy gap and exchange energy stabilization explains the deviations observed in Cr and Cu.

The inner transition elements consist of two series: the lanthanoids and the actinoids. The lanthanoid series involves the progressive filling of 4f orbitals and contains fourteen elements from Cerium (Ce) to Lutetium (Lu). Similarly, the actinoid series involves the progressive filling of 5f orbitals and contains fourteen elements from Thorium (Th) to Lawrencium (Lr). Therefore, Statements 1 and 3 are correct. Statement 2 is incorrect because Lanthanum has the electronic configuration [Xe]5d¹6s² and does not possess a 4f electron in its ground state. Statement 4 is incorrect because inner transition elements are placed separately at the bottom of the periodic table rather than in the main body. This separate placement helps maintain the compact structure of the periodic table.

The lanthanoid series corresponds to the progressive filling of the 4f subshell. Since an f-subshell contains seven orbitals and each orbital can accommodate two electrons, the maximum number of electrons that can occupy the 4f subshell is fourteen. Consequently, fourteen elements are formally classified as lanthanoids, extending from Cerium (Ce) to Lutetium (Lu). These elements exhibit similar chemical properties due to their closely related electronic configurations. They are placed in a separate row below the main periodic table and collectively constitute the first series of inner transition elements. Therefore, the lanthanoid series formally contains fourteen elements.

According to the IUPAC definition, a transition element is one whose atom or at least one of its common oxidation states contains an incompletely filled d-subshell. Scandium has the electronic configuration [Ar]3d¹4s² and therefore possesses a partially filled d-orbital in its ground state. This satisfies the definition of a transition element. Zinc, however, has the configuration [Ar]3d¹⁰4s². When it forms its common Zn²⁺ ion, the configuration becomes [Ar]3d¹⁰, which still contains a completely filled d-subshell. Since neither the atom nor its common ion contains an incompletely filled d-subshell, zinc does not qualify as a transition element under the strict IUPAC definition. This difference in electronic configuration explains the contrasting classifications of Sc and Zn.

Transition elements are grouped into different series according to the d-orbital being filled. Scandium is the first element of the 3d transition series. Yttrium occupies a similar position in the second transition series and belongs to the 4d series. Hafnium is part of the 5d transition series that follows the lanthanoid elements. Copernicium belongs to the 6d transition series, representing one of the heaviest known transition elements. Correctly associating these elements with their respective series requires understanding their positions in the periodic table and the orbital filling pattern that characterizes each transition series. Therefore, the correct matching is Y → 4d, Hf → 5d, Cn → 6d and Sc → 3d.

The third transition series extends from Lanthanum (La) to Mercury (Hg). However, after Lanthanum, the sequence is interrupted by fourteen lanthanoid elements in which the 4f orbitals are progressively filled. These elements occupy atomic numbers 58 to 71 and are collectively known as the lanthanoid series. After completion of the 4f filling, the 5d series resumes with Hafnium (Hf). This interruption creates an apparent discontinuity between La and Hf in the periodic table. The placement of lanthanoids in a separate row helps maintain the compact structure of the periodic table. Therefore, the discontinuity is caused by the insertion of the 4f-series elements between La and Hf.

The fourth transition series is known as the 6d series. It begins with Actinium (Ac) and, after the interruption caused by the actinoid series, continues from Rutherfordium (Rf) to Copernicium (Cn). Copernicium is the final element of this transition series and therefore signifies its completion. Like the earlier transition series, the 6d series is characterized by the progressive filling of d-orbitals. The identification of Copernicium as the terminal element of the 6d series helps classify the modern periodic table into four transition series. Therefore, Copernicium correctly represents the end of the 6d transition series.

Zinc, Cadmium, Mercury and Copernicium belong to Group 12 of the periodic table. Their general outer electronic configuration is (n−1)d¹⁰ns². In the ground state, all these elements possess completely filled d-orbitals, making Statements 1 and 2 correct. When these elements form their most common oxidation state of +2, the two ns electrons are removed while the d¹⁰ configuration remains intact. Consequently, they do not possess partially filled d-orbitals in their common oxidation states. According to the IUPAC definition, a transition element must have an incomplete d-subshell in either its atom or one of its common oxidation states. Therefore, these elements are generally excluded from the strict definition of transition elements.

When Group 12 elements such as Zn, Cd and Hg undergo ionization, electrons are removed first from the outermost ns orbital rather than from the inner d-subshell. As a result, the common +2 oxidation state is formed by losing the two ns electrons while retaining the completely filled (n−1)d¹⁰ configuration. Because the d-subshell remains fully occupied, these ions do not possess the incompletely filled d-orbitals required by the IUPAC definition of transition elements. Consequently, ionization does not alter their classification. The stability of the d¹⁰ configuration persists in their common oxidation states and prevents them from being considered true transition metals under the strict definition.

Historically, transition elements were named because they provide a transition in properties between the highly electropositive s-block metals and the more electronegative p-block elements. The s-block elements readily lose electrons and therefore exhibit strong metallic character. The d-block elements occupy an intermediate position and display properties between these two extremes. The p-block contains many non-metals and metalloids with comparatively higher electronegativity and non-metallic character. Therefore, when arranged in increasing order of electronegativity or non-metallic character, the correct sequence is s-block, followed by d-block, followed by p-block. This historical interpretation explains the origin of the term "transition elements."

The chemistry of transition elements can be understood using valence concepts because the energies of the (n−1)d and ns orbitals are very close to one another. As a result, electrons from both sets of orbitals can participate in bond formation and ionization. This explains the occurrence of multiple oxidation states, which is a characteristic feature of transition elements. Unlike representative elements, where valence is often fixed, transition metals can exhibit several oxidation states due to the availability of both d and s electrons. Nevertheless, these oxidation states follow predictable patterns that can be interpreted using the principles of valence and electronic configuration. Therefore, the comparable energies and overlap of the (n−1)d and ns orbitals provide the fundamental basis for applying valence concepts to transition elements.

Iron is one of the most important transition metals in human civilization and modern industry. It belongs to the first transition series (3d series) and serves as the principal component of steel, one of the most widely used engineering materials. Iron and its alloys are used extensively in construction, transportation, machinery, infrastructure and manufacturing industries. The strength, durability and versatility of steel have made iron indispensable for industrial development. NCERT specifically highlights iron as a major industrial metal among transition elements. Its abundance and favourable mechanical properties have contributed significantly to technological progress and economic growth worldwide.

Titanium is classified as an industrial transition metal because of its extensive technological and engineering applications. It is valued for its high strength-to-weight ratio, excellent corrosion resistance and durability. Titanium is widely used in aerospace structures, marine equipment, chemical processing industries and medical implants. In contrast, silver, gold and platinum are commonly categorized as precious metals because of their rarity, high economic value and use in jewellery, currency and specialized applications. NCERT identifies titanium as an important industrial metal among transition elements. Therefore, Titanium is correctly categorized as an industrial metal rather than a precious metal.

Silver, gold and platinum have historically been used for jewellery, ornaments and currency due to their lustre, rarity and resistance to corrosion. Uranium, Thorium and Protactinium belong to the actinoid series and are important sources of nuclear energy. Zinc, Cadmium and Mercury are end members of their respective transition series and are generally excluded from the strict IUPAC definition of transition elements because of their filled d¹⁰ configurations. Iron and Titanium are major industrial metals that contribute significantly to infrastructure, engineering and technological development. Matching these applications with their corresponding elements gives the sequence 1-b, 2-c, 3-d and 4-a.

Thorium and Uranium belong to the actinoid series, which consists of inner transition elements characterized by the progressive filling of 5f orbitals. These elements are well known for their radioactive nature and their ability to release enormous amounts of energy through nuclear reactions. Consequently, they are widely used as fuels in nuclear reactors and are regarded as excellent sources of nuclear energy. Statement 3 is incorrect because bronze was produced primarily from copper and tin rather than actinoid elements. Statement 4 is also incorrect because actinoids involve the filling of 5f orbitals, not 4f orbitals. Therefore, only Statements 1 and 2 are correct.