Consider the following elements In, Tl, Al, Pb, Sn and Ge. The most stable oxidation states of elements with highest and lowest first ionisation enthalpies, respectively, are

### Related Formula Basic strength order among Group 15 hydrides drops down the group due to an increase in size and a decrease in charge density: NH_3 > PH_3 > AsH_3 > SbH_3 > BiH_3$$NH_3 > PH_3 > AsH_3 > SbH_3 > BiH_3$$ ### Core Logic Let's analyze the properties specified: 1. The element must be able to form dpi-dpi$d\pi-d\pi$ bonds with transition metals. Nitrogen cannot form these bonds because it lacks vacant d$d$-orbitals in its valence shell. Therefore, nitrogen is excluded. 2. Among the remaining elements (Phosphorus, Arsenic, Antimony, Bismuth) that contain available d$d$-orbitals, basic strength decreases down the group. Phosphorus forms phosphine (PH_3$PH_3$), which is the strongest base among the remaining members. ### Step 1: Identify the Atomic Number The identified element is Phosphorus (P$P$). The atomic number of Phosphorus is 15$15$. ### Pattern Recognition Pay attention to qualifying statements like *'among elements that form dpi-dpi$d\pi-d\pi$ bonds'*. This explicitly excludes second-period elements (like Nitrogen), making Phosphorus (Z=15$Z=15$) the top choice for basicity. ### Evaluation Rubric / Model Answer null ### Chapter Mix Class 12 Chemistry: p-Block Elements

### Core Logic NH_3$NH_3$ undergoes extensive intermolecular hydrogen bonding due to the high electronegativity and small size of Nitrogen. PH_3$PH_3$ (Phosphine) molecules are only held together by weak van der Waals (dispersion) forces because Phosphorus is less electronegative and larger, unable to form strong hydrogen bonds. ### Step 1: Evaluate Statements Assertion (A) is correct: PH_3$PH_3$ has a lower boiling point than NH_3$NH_3$ because breaking H-bonds in NH_3$NH_3$ requires more energy. Reason (R) is incorrect: It falsely claims NH_3$NH_3$ has van der Waals association and PH_3$PH_3$ has hydrogen bonding. It is exactly the opposite. ### Pattern Recognition N, O, and F are the only atoms electronegative enough to form stable hydrogen bonds in simple hydrides. Boiling point anomaly: NH_3 > PH_3$NH_3 > PH_3$ purely due to H-bonding in NH_3$NH_3$. ### Evaluation Rubric / Model Answer null ### Chapter Mix Class 12 Chemistry: The p-Block Elements Class 11 Chemistry: Chemical Bonding and Molecular Structure

### Related Formula \text{Anomalous properties of second-period elements.} ### Core Logic The anomalous properties of oxygen compared to other chalcogens stem directly from its position in the second period of the periodic table. It is characterized by: 1. An exceptionally **small atomic radius**. 2. Highly pronounced **electronegativity**. 3. Complete absence of low-energy valence d$d$-orbitals. ### Step 1: Selection Verification Therefore, the combination of small size and high electronegativity is the correct choice, matching option (3). ### Pattern Recognition All first members of periodic blocks (textN$\text{N}$, textO$\text{O}$, textF$\text{F}$) deviate significantly from their heavier group members due to their high charge density, high electronegativity, and lack of d$d$-orbitals. ### Evaluation Rubric / Model Answer null ### Chapter Mix Class 11 Chemistry: p-Block Elements

### Core Logic Boron forms a highly compact, robust icosahedral covalent polymeric three-dimensional framework structure (B_12$B_{12}$ units). This extremely solid, dense crystalline lattice organization requires immense thermal activation energy to rupture, explaining why its melting point (2453text K$2453\text{ K}$) is uniquely elevated among Group 13 elements. Both statements are true and (R) is the perfect explanation. ### Chapter Mix Class 11 Chemistry: p-Block Elements

### Step 1: Audit oxidation numbers individually ### Step 2: Sum the targets The compounds displaying an exact +4$+4$ assignment are textH_2textSO_3$\text{H}_2\text{SO}_3$, textSOCl_2$\text{SOCl}_2$, and textSF_4$\text{SF}_4$. The total number is 3. ### Pattern Recognition Sulfurous derivatives, thionyl groupings, and tetrafluoride configurations typically feature the +4$+4$ oxidation level state. ### Chapter Mix Class 11 Chemistry: Redox Reactions Class 12 Chemistry: p-Block Elements