Electronic configuration of four elements A, B, C and D are given below : (A) 1mathrms^2 2mathrms^2 2mathrmp^3$1\mathrm{s}^2 2\mathrm{s}^2 2\mathrm{p}^3$ (B) 1mathrms^2 2mathrms^2 2mathrmp^4$1\mathrm{s}^2 2\mathrm{s}^2 2\mathrm{p}^4$ (C) 1mathrms^2 2mathrms^2 2mathrmp^5$1\mathrm{s}^2 2\mathrm{s}^2 2\mathrm{p}^5$ (D) 1mathrms^2 2mathrms^2 2mathrmp^2$1\mathrm{s}^2 2\mathrm{s}^2 2\mathrm{p}^2$ Which of the following is the correct order of increasing electronegativity (Pauling's scale)?

### Related Formula Electron Gain Enthalpy (Delta_egH$\Delta_{eg}H$) trends in Group 16 and Group 17 elements. ### Core Logic Evaluating both statements: * **Statement I is false**: Due to its small size, strong inter-electronic repulsions in the compact 2p$2p$ subshell of Fluorine limit incoming electrons compared to Chlorine. As a result, Chlorine (textCl$\text{Cl}$) has the *most negative* electron gain enthalpy in Group 17. * **Statement II is true**: Similarly, the exceptionally small size of the Oxygen atom creates intense electron-electron repulsions. Consequently, it has the *least negative* electron gain enthalpy among all elements in Group 16. ### Step 1: Final Assessment Therefore, Statement I is false while Statement II is true, matching choice (4). ### Pattern Recognition Third-period elements (textCl$\text{Cl}$, textS$\text{S}$) exhibit more negative electron gain enthalpies than their second-period counterparts (textF$\text{F}$, textO$\text{O}$) due to lower inter-electronic repulsion in their larger valence shells. ### Evaluation Rubric / Model Answer null ### Chapter Mix Class 11 Chemistry: Classification of Elements and Periodicity

### Core Logic Fluorine is the most electronegative element and lacks vacant d-orbitals in its valence shell. Consequently, it exhibits only a -1$-1$ oxidation state (and 0$0$ in elemental form) and cannot show variable positive oxidation states unlike other halogens. ### Pattern Recognition First element of a group lacks d-orbitals rightarrow$\rightarrow$ anomalous properties rightarrow$\rightarrow$ Fluorine only exhibits -1$-1$. ### Chapter Mix Class 11 Chemistry: Classification of Elements and Periodicity in Properties

### Core Logic Statement I is true: The lanthanides (4textf$4\text{f}$) and actinides (5textf$5\text{f}$) 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 (Z_texteff$Z_{\text{eff}}$) dominates, pulling valence electrons tighter, increasing ionisation energy. ### Evaluation Rubric / Model Answer null ### 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., Na_2O$Na_2O$). Group 17 elements are non-metals and form acidic oxides (e.g., Cl_2O_7$Cl_2O_7$). 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. ### Evaluation Rubric / Model Answer null ### Chapter Mix Class 11 Chemistry: Classification of Elements and Periodicity in Properties