Which of the following statements is wrong ?
The stability of hydrides decreases from NH3 to BiH3 which can be observed from their bond dissociation enthalpy. The correct order is NH3 < PH3 < AsH3 < SbH3 < BiH3.
Property NH3 PH3 AsH3 SbH3 BiH3
Bond energy HΘ(E–H) / kJ mol–1 389 322 297 255 ––
We are asked: "Which of the following statements is wrong?" and given four options. We need to evaluate each one for correctness.
Statement: "The stability of hydrides increases from NH3 to BiH3 in group 15 of the periodic table."
Concept: In Group 15, the elements are N, P, As, Sb, Bi. The stability of their hydrides (EH3) depends on the E-H bond strength. As we move down the group, the atomic size increases, and the effective overlap of the orbital forming the E-H bond decreases. This makes the bond weaker. Therefore, the stability of hydrides decreases down the group. NH3 is the most stable, and BiH3 is the least stable and highly unstable.
Conclusion: This statement is WRONG. The stability actually decreases.
Statement: "Single N – N bond is weaker than the single P – P bond."
Concept: The bond dissociation energy for a single N-N bond (in N2H4) is about 159 kJ/mol, while for a single P-P bond (in P4) it is about 209 kJ/mol. This is because smaller nitrogen atoms have high interelectronic repulsion between the lone pairs of electrons on adjacent atoms, which weakens the N-N bond. Larger phosphorus atoms have less repulsion, leading to a stronger single bond.
Conclusion: This statement is CORRECT. The N-N single bond is indeed weaker.
Statement: "N2O4 has two resonance structures."
Concept: Dinitrogen tetroxide (N2O4) exists in equilibrium with NO2. Its structure can be represented by two major resonance forms where the N-N bond is single, and the negative charge is delocalized between the two oxygen atoms of each NO2 group. The two equivalent resonance structures are shown below:
These structures explain the properties of the molecule, such as its long N-N bond length, which is characteristic of a bond with a partial double bond character due to resonance.
Conclusion: This statement is CORRECT.
Statement: "Nitrogen cannot form dπ – pπ bond."
Concept: A dπ-pπ bond requires the involvement of d-orbitals. Nitrogen (atomic number 7) has the electron configuration . Its principal quantum number is n=2, which does not have d-orbitals (d-orbitals start from n=3). Therefore, nitrogen cannot use d-orbitals to form dπ-pπ bonds. Heavier elements in the group (like phosphorus) can form such bonds because they have vacant d-orbitals in their valence shell.
Conclusion: This statement is CORRECT.
After evaluating all four statements, only Option 1 is incorrect. The stability of Group 15 hydrides decreases down the group, it does not increase.
The wrong statement is: "The stability of hydrides increases from NH3 to BiH3 in group 15 of the periodic table."
Bond Dissociation Energy (D): The energy required to break a bond homolytically.
Lone Pair-Lone Pair Repulsion: A key reason for the weakness of N-N, O-O, and F-F single bonds compared to their subsequent group members.
dπ-pπ Bonding: Occurs when an atom with filled p-orbitals donates electron density to the vacant d-orbitals of another atom. This is common in compounds of elements in the 3rd period and below (e.g., in , ).