Given below are two statements: Statement I: Fluorine has most negative electron gain enthalpy in its group. Statement II: Oxygen has least negative electron gain enthalpy in its group. In the light of the above statements, choose the most appropriate from the options given below.

Core Logic

Statement I is true: The lanthanides (4f$4\text{f}$) and actinides (5f$5\text{f}$) are separated at the bottom of the periodic table to avoid distortion of the periodic trends layout and preserve clean structural classification groups.

Statement II is false: s-block elements (alkali and alkaline earth metals) possess exceptionally low ionization energies, making them highly reactive; thus, they are always encountered as compounds rather than in native pure forms in nature.

Chapter Mix

Class 11 Chemistry: Classification of Elements and Periodicity in Properties

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