CUET UG Chemistry Booster Test – 2 Introduction & Classification

The general electronic configuration of d-block elements is represented as (n−1)d¹⁻¹⁰ ns¹⁻². This configuration arises because the differentiating electron enters the d-subshell of the penultimate shell. Palladium is a notable exception and possesses the configuration 4d¹⁰ 5s⁰ instead of the expected arrangement. Another important feature of d-block elements is the small energy difference between the (n−1)d and ns orbitals. Because these orbitals have comparable energies, electrons can shift between them, resulting in irregular configurations and variable oxidation states. Statement 3 is incorrect because (n−1) refers to the penultimate energy level, not specifically to p orbitals. Therefore, Statements 1, 2 and 4 are correct.

The modern periodic table is divided into s-, p-, d- and f-blocks based on the subshell into which the differentiating electron enters. The s-block elements occupy the extreme left side of the periodic table and include Groups 1 and 2. The d-block elements occupy the central portion of the table and extend from Groups 3 to 12. The p-block elements are located on the right side and include Groups 13 to 18. Therefore, when moving from left to right across the periodic table, the order is s-block → d-block → p-block. This arrangement reflects the sequence of orbital filling and is fundamental to understanding periodic trends and electronic configurations.

The classification of elements into blocks is based on the subshell into which the differentiating electron enters. In the f-block elements, electrons progressively fill the 4f and 5f orbitals, producing the lanthanoid and actinoid series. In contrast, d-block elements involve the filling of 3d, 4d, 5d and 6d orbitals of the penultimate energy level. This difference in orbital filling accounts for many of the distinct chemical and physical properties of these two groups of elements. The f-block elements are known as inner transition metals, while the d-block elements are generally referred to as transition metals. Therefore, the correct comparison is that f-block elements fill 4f and 5f orbitals whereas d-block elements fill d orbitals of the penultimate shell.

The f-block elements include the lanthanoid and actinoid series. If these elements were placed in their actual positions within the periodic table, the table would become excessively wide and difficult to display. To preserve the overall shape and compactness of the periodic table, these elements are placed in a separate panel at the bottom. This arrangement allows the periodic table to remain practical while still accurately representing the electronic structure of the elements. The separate panel contains the 4f and 5f series, which are collectively known as inner transition elements. Therefore, the structural feature used to accommodate these elements is a separate panel placed below the main body of the periodic table.

Scandium has the electronic configuration [Ar] 3d¹ 4s². According to the IUPAC definition, a transition element is one that has an incomplete d subshell in its atom or forms at least one ion with an incomplete d subshell. Since Scandium possesses a partially filled 3d orbital in its ground state, it satisfies this definition and is classified as a transition element. The presence of an incompletely filled d subshell is the key criterion used by IUPAC. Although Sc³⁺ has the configuration [Ar], the neutral atom itself already possesses a partially filled d subshell. Therefore, Scandium is regarded as a transition metal.

The fundamental feature that distinguishes transition and inner transition elements from non-transition elements is the presence of partially filled d or f orbitals in their atoms or ions. The presence of these orbitals gives rise to several characteristic properties such as variable oxidation states, coloured compounds, magnetic behaviour, complex formation and catalytic activity. These properties are not commonly observed in s- and p-block elements. Because of their unique electronic configurations and chemical behaviour, transition elements require separate study. The d-block elements involve progressive filling of d-orbitals, while the f-block elements involve progressive filling of f-orbitals. These special electronic arrangements influence their physical and chemical properties significantly and make them an important part of inorganic chemistry.

The lanthanoids are the series of inner transition elements in which the 4f orbitals are progressively filled. According to NCERT, the lanthanoid series extends from Cerium (Ce) to Lutetium (Lu). These elements are placed separately at the bottom of the periodic table along with the actinoids. The lanthanoids exhibit similar chemical properties because of their comparable electronic configurations. They are often referred to as rare earth elements and are characterized by the gradual filling of the 4f subshell. The passage explicitly identifies the lanthanoids as the 4f series extending from Ce to Lu. Therefore, Option B correctly represents the boundaries of the lanthanoid series.

The series extending from Thorium (Th) to Lawrencium (Lr) is known as the actinoid series. These elements belong to the f-block and are characterized by the progressive filling of 5f orbitals. Actinoids are inner transition elements and are generally radioactive in nature. Their electronic configurations involve the gradual addition of electrons to the 5f subshell, resulting in unique nuclear and chemical properties. The actinoids are placed separately at the bottom of the periodic table to maintain its compact structure. Since the defining feature of this series is the filling of 5f orbitals, Option C is the correct answer.

The first transition series extends from Scandium (Sc) to Zinc (Zn). The d-subshell contains five orbitals, and each orbital can accommodate a maximum of two electrons according to the Pauli exclusion principle. Consequently, a complete d-subshell can hold ten electrons. As the 3d orbitals are progressively filled across the series, ten elements are theoretically required to complete the filling process. This is why the first transition series consists of ten elements, beginning with Scandium and ending with Zinc. The same principle applies to the 4d, 5d and 6d transition series as well. Therefore, the correct answer is ten elements.

The transition series are classified according to the d-orbital being filled. The first transition series or 3d series extends from Scandium (Sc) to Zinc (Zn). The second transition series or 4d series extends from Yttrium (Y) to Cadmium (Cd). The third transition series or 5d series extends from Lanthanum (La) through Hafnium (Hf) to Mercury (Hg), with the lanthanoids occurring in between. The fourth transition series or 6d series begins with Actinium (Ac), continues after the actinoid series and extends from Rutherfordium (Rf) to Copernicium (Cn). Matching these boundary elements correctly gives the answer 1-d, 2-c, 3-b and 4-a.

The third transition series, commonly called the 5d series, begins with Lanthanum (La) and extends up to Mercury (Hg). After Lanthanum, there is a discontinuity in the sequence because the fourteen lanthanoid elements are inserted. These elements involve the filling of 4f orbitals and belong to the f-block. Once the lanthanoid series is completed, the filling of 5d orbitals resumes with Hafnium (Hf). This arrangement explains the apparent gap in atomic numbers between La and Hf. The series finally terminates at Mercury (Hg), which possesses a completely filled d-subshell. Transition elements are characterized primarily by the filling of d-orbitals, while the outermost ns orbitals also participate in electron configuration. Therefore, it is incorrect to state that the series involves the filling of only the 6s orbitals. The structure of the 5d series demonstrates the relationship between d-block and f-block elements and is an important feature of periodic classification.

Actinium (Ac) is associated with the beginning of the fourth transition series, which is often referred to as the 6d series. However, after Actinium, the next fourteen elements belong to the actinoid series where the 5f orbitals are progressively filled. These elements constitute the f-block and are placed separately in the periodic table for convenience and clarity. After completion of the actinoid series, the filling of 6d orbitals resumes with Rutherfordium (Rf). This pattern is analogous to the interruption observed in the 5d series by the lanthanoids. The arrangement reflects the order of orbital filling predicted by quantum mechanical principles and helps maintain the compact structure of the periodic table. Thus, the discontinuity in atomic numbers is not due to decay processes or unusual chemical behavior but results from the insertion of the actinoid series.

According to the IUPAC definition, a transition element is one that possesses an incomplete d-subshell in its atom or in one of its common oxidation states. Zinc, Cadmium and Mercury have completely filled d10 configurations in both their atoms and commonly occurring ions. Therefore, they do not strictly qualify as transition elements. Nevertheless, these elements are traditionally studied along with transition metals because they occur at the end of the 3d, 4d and 5d transition series respectively. Their chemical properties, periodic placement and historical association connect them closely with the transition elements. Studying them together provides a complete understanding of trends across the d-block. Hence, their inclusion is based primarily on their position as terminal members of the transition series rather than on satisfaction of the formal IUPAC definition.

The end members of the transition series correspond to the completion of successive d-subshells. Zinc is the terminal member of the 3d series, Cadmium completes the 4d series, Mercury completes the 5d series and Copernicium is associated with the completion of the 6d series. Since the question asks for decreasing order of principal quantum number, the element associated with the highest d-level must come first. Therefore, Copernicium (6d) precedes Mercury (5d), which is followed by Cadmium (4d) and then Zinc (3d). This sequence reflects the progressive increase in principal quantum number from one transition series to the next. Understanding the arrangement of transition series is essential for interpreting periodic trends and electronic configurations in the d-block.

The term "transition metals" was introduced because these elements exhibit properties that are intermediate between the highly electropositive metals of the s-block and the more electronegative non-metals of the p-block. Historically, chemists observed that these elements showed gradual changes in physical and chemical properties across the periodic table. Their variable oxidation states, tendency to form coloured compounds, complex formation and catalytic behavior distinguished them from representative elements. Because they appeared to provide a transition in properties between the two major groups of elements, they were collectively termed transition metals. Although modern quantum mechanics explains their behavior through the filling of d-orbitals, the historical name remains widely used. The terminology reflects the important role of these elements in connecting different regions of the periodic table.

Transition elements exhibit several characteristics that distinguish them from s- and p-block elements, such as variable oxidation states, coloured ions, complex formation and catalytic activity. Because of these distinctive features, they are usually studied separately in chemistry. However, this does not imply that the theory of valence completely fails for transition metals. The fundamental concepts of valence and bonding remain applicable, although the presence of partially filled d-orbitals introduces additional complexity. Modern bonding theories, including valence bond theory and crystal field theory, successfully explain many properties of transition elements. Therefore, Statement 2 is correct because transition elements are commonly treated as a separate group for systematic study. Statement 3 is also correct because the theory of valence can be extended to explain the behavior of transition elements. Statements 1 and 4 are incorrect because bonding theories exist and the concept of valence remains meaningful for transition metals.

Titanium is one of the most important transition metals of the 3d series. It is widely used in aerospace engineering, chemical industries, medical implants and marine applications because of its high strength-to-weight ratio and exceptional resistance to corrosion. Along with iron and copper, titanium represents a technologically significant transition metal that has contributed greatly to modern industrial development. Its compounds are also important, particularly titanium dioxide, which is extensively used as a white pigment. Transition metals are known for their utility in engineering and manufacturing due to their metallic bonding, mechanical strength and diverse oxidation states. Titanium exemplifies these characteristics and is therefore highlighted among industrially important transition elements. Silver and platinum are valuable precious metals, while uranium is an actinoid rather than a transition metal of the 3d series.

Iron is one of the most extensively used industrial transition metals due to its abundance, strength and importance in steel production. Platinum is classified as a precious transition metal because of its rarity, high value and extensive use in catalysis and jewellery. Uranium belongs to the actinoid series, making it an inner transition element and an important source of nuclear energy. Zinc, although placed in the d-block, possesses a completely filled d10 configuration and therefore does not satisfy the strict IUPAC definition of a transition element. It is traditionally studied with transition metals because it is the terminal member of the 3d series. Correctly matching these categories requires knowledge of periodic classification and practical applications of these elements. The given matching accurately reflects their respective positions and significance in chemistry.

The NCERT discussion highlights four transition metals that have played especially important roles in the development of human civilization: iron, copper, silver and gold. Copper was among the earliest metals used by humans and contributed to the Bronze Age. Iron revolutionized tools, agriculture and construction, leading to the Iron Age. Silver and gold have long been valued for currency, ornamentation and trade. These metals significantly influenced technological progress, economic systems and cultural development throughout history. Their extraction, processing and utilization marked major milestones in human advancement. NCERT emphasizes these metals because they illustrate the practical importance of transition elements beyond their chemical properties. The explicit mention of four such metals makes Option B the correct answer.

Thorium (Th), Protactinium (Pa) and Uranium (U) belong to the actinoid series, which forms part of the inner transition elements. These elements involve the progressive filling of 5f orbitals and are located in the f-block of the periodic table. Uranium and thorium are particularly important because they serve as major fuels in nuclear reactors. Their radioactive nature enables them to undergo nuclear fission, releasing enormous amounts of energy. The actinoids exhibit unique electronic configurations and nuclear properties that distinguish them from d-block transition metals and representative elements. Because of their importance in nuclear technology, these elements are frequently discussed in relation to energy production. NCERT specifically identifies them as actinoids and excellent sources of nuclear energy. Therefore, they are correctly classified as inner transition elements.