CBSE Class 11 Chemistry Chapter 3: Classification of Elements and Periodicity NCERT Solutions

Solution: The fundamental principle guiding the organization of elements in the periodic table is their classification into periods and groups based on their chemical and physical properties. This systematic arrangement simplifies the study of elements and their compounds by placing elements with similar characteristics in the same group, highlighting periodic trends.

Solution: Dmitri Mendeleev primarily used the atomic weight (or atomic mass) of elements as the basis for classifying them in his periodic table. He arranged the elements in increasing order of their atomic weights, placing elements with similar properties into vertical columns called groups.

However, Mendeleev did not strictly adhere to this principle throughout his table. He recognized that some elements did not fit the pattern if arranged solely by atomic weight. To maintain the grouping of elements with similar properties, he occasionally reversed the order of elements with slightly different atomic weights. A notable example is the placement of Tellurium (atomic weight 127.6) before Iodine (atomic weight 126.9). He placed Tellurium in Group VI and Iodine in Group VII because Iodine's properties were more similar to Fluorine, Chlorine, and Bromine in Group VII.

Solution: The core difference lies in the fundamental property used for classification. Mendeleev's Periodic Law stated that the physical and chemical properties of elements are periodic functions of their atomic weights. In contrast, the Modern Periodic Law, established later, states that these properties are periodic functions of their atomic numbers. The atomic number, representing the number of protons, provides a more fundamental basis for periodicity.

Solution: The period number in the periodic table corresponds to the principal quantum number (n) for the outermost electron shell being filled. For the sixth period, the principal quantum number is $n\; =\; 6$.

According to the rules of quantum mechanics, the azimuthal quantum number ($l$) can take integer values from 0 to $n-1$. For $n=6$, the possible values of $l$ are 0, 1, 2, and 3, which correspond to the 6s, 6p, 6d, and 6f subshells, respectively. However, based on the Aufbau principle and experimental observations of energy levels, the filling order in the sixth period involves the 6s, 4f, 5d, and 6p subshells.

Let's count the number of orbitals available in these subshells:

• The 6s subshell has 1 orbital (for $l=0$).
• The 4f subshell has 7 orbitals (for $l=3$).
• The 5d subshell has 5 orbitals (for $l=2$).
• The 6p subshell has 3 orbitals (for $l=1$).

The total number of orbitals available for filling electrons in the sixth period is the sum of orbitals in these subshells: $1\; +\; 7\; +\; 5\; +\; 3\; =\; 16$ orbitals.

According to the Pauli's exclusion principle, each atomic orbital can accommodate a maximum of two electrons with opposite spins. Therefore, the total number of electrons that can be accommodated in the 16 orbitals is $16\; \backslash times\; 2\; =\; 32$ electrons.

Each electron corresponds to one element in the period. Hence, the sixth period of the periodic table should contain 32 elements.