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

Solution: The fundamental principle guiding the organisation of elements in the periodic table is to arrange them in a systematic manner based on their properties. Elements are classified into periods (rows) and groups (columns). This arrangement ensures that elements with similar chemical and physical properties are placed in the same group, making the study of their behaviour and compounds more manageable and logical.

Solution: Dmitri Mendeleev primarily used the atomic weight (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 in all cases. He recognised that grouping elements with similar properties was more important for establishing a coherent periodic law. In certain instances, to fit elements into appropriate groups based on their properties, he placed an element with a slightly higher atomic weight before an element with a lower atomic weight. A notable example is the placement of Tellurium (atomic weight 127.6) before Iodine (atomic weight 126.9). This was done because Iodine's chemical properties are more similar to other halogens (like Fluorine, Chlorine, Bromine) in Group VII, despite its lower atomic weight compared to Tellurium.

Solution: The fundamental difference between Mendeleev's Periodic Law and the Modern Periodic Law lies in the property used as the basis for classification.

Mendeleev's Periodic Law stated that the physical and chemical properties of the elements are periodic functions of their atomic weights. This meant that as atomic weights increased, properties repeated at regular intervals.

The Modern Periodic Law, on the other hand, states that the physical and chemical properties of the elements are periodic functions of their atomic numbers. The atomic number, which represents the number of protons in an atom's nucleus, is a more fundamental property and provides a more accurate basis for classification, resolving many of the anomalies found in Mendeleev's system.

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

According to the rules of quantum mechanics, for a given value of $n$, the azimuthal quantum number ($l$) can take integer values from 0 to $n-1$. Thus, for $n\; =\; 6$, the possible values of $l$ are 0, 1, 2, 3, and 4. These values correspond to the subshells 6s ($l=0$), 6p ($l=1$), 6d ($l=2$), and 6f ($l=3$). Note that the 6g subshell ($l=4$) is theoretically possible but is not filled in the sixth period according to the Aufbau principle and experimental observations; instead, the 4f and 5d subshells are filled before the 6p subshell.

The subshells that are filled in the sixth period are 6s, 4f, 5d, and 6p. Let's determine the number of orbitals available in each:

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

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 these 16 orbitals is $16\; \backslash times\; 2\; =\; 32$ electrons.

Hence, the sixth period of the periodic table can accommodate a maximum of 32 elements, corresponding to the filling of these 32 available electronic positions.