Given below are two statements: Statement (I): The 4f and 5f series of elements are placed separately in the Periodic table to preserve the principle of classification. Statement (II) : S-block elements can be found in pure form in nature. In the light of the above statements, choose the most appropriate answer from the options given below :

Core Logic

Assertion A states that the first ionisation enthalpy decreases across a period. This is false, because first ionisation energy generally increases along a period from left to right.

Reason R states that the increasing nuclear charge outweighs the shielding across the period. This is true, and it is the exact reason why the atomic radius decreases and ionisation enthalpy increases across a period.

Step 1: Final Conclusion

Since the assertion is incorrect and the reason is a factually correct statement, A is false but R is true.

Pattern Recognition

Recall the horizontal periodic trends: across a period, effective nuclear charge (Zeff$Z_{\text{eff}}$) dominates, pulling valence electrons tighter, increasing ionisation energy.

Chapter Mix

Class 11 Chemistry: Classification of Elements and Periodicity in Properties

Core Logic

Statement I: Along a period, the chemical reactivity of elements first decreases and then increases. Group 1 elements (alkali metals) are highly reactive, Group 17 elements (halogens) are also highly reactive, but the elements in the middle (like carbon) are less reactive. Moreover, Group 18 elements (noble gases) are mostly inert. Therefore, statement I is false.

Statement II: Group 1 elements are metals and form basic oxides (e.g., Na₂O$Na_2O$). Group 17 elements are non-metals and form acidic oxides (e.g., Cl₂O₇$Cl_2O_7$). Therefore, statement II is true.

Pattern Recognition

Reactivity is highest at the extreme left (Group 1) and extreme right (Group 17) of the periodic table, excluding noble gases. Metallic character = basic oxides; Non-metallic character = acidic oxides.

Chapter Mix

Class 11 Chemistry: Classification of Elements and Periodicity in Properties

Core Logic

Decode the IUPAC name "Unununnium" into its atomic number. Un = 1 Un = 1 Un = 1 Suffix = ium So, the atomic number is 111.

Step 1: Finding group number

For d-block elements with Z > 100, the group number is the last two digits of the atomic number. Since Z = 111, the element belongs to group 11 (the Copper group).

Chapter Mix

Class 11 Chemistry: Classification of Elements and Periodicity in Properties

Core Logic

From the grid layout, A^$A^\prime$ and B^$B^\prime$ are in the same period (left to right), while C^$C^\prime$ and D^$D^\prime$ are below them in the next period. In general, moving along a period from left to right, size decreases and metallic character decreases. Moving down a group, size increases and metallic character increases.

Atomic Radii / Size: B^ < A^$B^\prime < A^\prime$ (across period) D^ < C^$D^\prime < C^\prime$ (across period) A^ < C^$A^\prime < C^\prime$ and B^ < D^$B^\prime < D^\prime$ (down the group) Combining them: B^ < A^ < D^ < C^$B^\prime < A^\prime < D^\prime < C^\prime$. (Statement A is correct, C is incorrect).

Metallic Character: B^ < A^$B^\prime < A^\prime$ and D^ < C^$D^\prime < C^\prime$ Overall trend follows size closely for these blocks: B^ < A^ < D^ < C^$B^\prime < A^\prime < D^\prime < C^\prime$. (Statement B is correct).

Ionic Radii (+$+$ ions): Follows the same basic trend as atomic radii: B⁺ < A⁺ < D⁺ < C⁺$B^{\prime +} < A^{\prime +} < D^{\prime +} < C^{\prime +}$. (Statement D is correct).

Step 1: Final Selection

Statements A, B, and D are correct. Statement C contradicts A, so it is incorrect. Option 2 is the right choice.

Chapter Mix

Class 11 Chemistry: Classification of Elements and Periodicity in Properties