Q4. Name a species isoelectronic with each of (i) F−, (ii) Ar, (iii) Mg2+, (iv) Rb+ (NCERT Exercise 3.11, page 96).
(i) F− has 10 electrons: Ne, Na+ or O2−. (ii) Ar has 18: Cl−, K+ or S2−. (iii) Mg2+ has 10: Na+, Ne or F−. (iv) Rb+ has 36: Kr, Br− or Sr2+.
Q5. Why is the first ionisation enthalpy of sodium lower than that of magnesium, while its second ionisation enthalpy is higher (NCERT Exercise 3.17, page 97)?
First: both lose a 3s electron, but magnesium has a larger nuclear charge and a smaller atom, so its electron is held more tightly. Second: Na+ is [Ne], so its second electron must come from the stable, closed neon core; Mg+ still has a 3s electron to lose.
Q6. The first ionisation enthalpies (kJ mol−1) of group 13 are B 801, Al 577, Ga 579, In 558, Tl 589. Explain the departures from a steady fall (NCERT Exercise 3.19, page 97).
B to Al falls as expected (larger atom). Ga is slightly above Al because its ten 3d electrons shield the outer electrons poorly, so the effective nuclear charge rises. Tl is above In for the same reason: the 4f and 5d electrons shield poorly. Order: B > Tl > Ga > Al > In; In falls as expected.
Q7. Would you expect the second electron gain enthalpy of oxygen to be positive, more negative or less negative than the first (NCERT Exercise 3.21, page 97)?
Positive. The second electron must be forced onto O−, which is already negative and repels it, so energy has to be supplied.
Q8. Would two isotopes of an element have the same first ionisation enthalpy (NCERT Exercise 3.25, page 97)?
Yes, practically the same: isotopes have the same nuclear charge and the same electronic configuration; only the neutron count differs. Small isotope shifts do exist because nuclear mass and size affect electronic energies, but they are negligible at this school-level precision.
Q9. How would you react to the statement that the electronegativity of nitrogen on the Pauling scale is 3.0 in all its compounds (NCERT Exercise 3.23, page 97)?
It is not true. Electronegativity is not a fixed, measurable property: it depends on the atoms bonded to nitrogen and on its state of bonding, so it varies from compound to compound. 3.0 is a typical value.
Q10. The table gives ΔiH1, ΔiH2 and ΔegH (kJ mol−1): I 520, 7300, −60; II 419, 3051, −48; III 1681, 3374, −328; IV 1008, 1846, −295; V 2372, 5251, +48; VI 738, 1451, +40. (NCERT prints −40 for VI; the sign is corrected here, as the answer explains.) Which is (a) the least reactive element, (b) the most reactive metal, (c) the most reactive non-metal, (d) the least reactive non-metal, (e) a metal forming a stable MX2 halide, (f) a metal forming a mainly covalent MX halide (NCERT Exercise 3.31, page 98)?
(a) V (highest ionisation enthalpy and positive ΔegH: helium). (b) II (potassium), following the intended alkali-metal trend for reactions such as with water; aqueous reducing strength is a different comparison. (c) III (high ionisation enthalpy, most negative ΔegH: fluorine). (d) Among the two reactive halogen non-metals III and IV, IV (iodine) is less reactive. If the question includes noble-gas V among all non-metals, V is the least reactive; the unqualified wording is ambiguous. (e) VI: its second ionisation enthalpy is only about twice the first, so it forms M2+ (magnesium). NCERT prints its ΔegH as −40, but ground-state electron attachment to magnesium is unfavourable: the filled 3s subshell gives way to an unbound additional electron in this elementary picture. The size, 40, is only an estimate, since Mg− is too unstable to measure. (f) I: a huge second ionisation enthalpy limits it to +1, and its small ion gives covalent character (lithium).
Q11. Predict the formulas of the stable binary compounds of (a) lithium and oxygen, (b) magnesium and nitrogen, (c) aluminium and iodine, (d) silicon and oxygen, (e) phosphorus and fluorine, (f) element 71 and fluorine (NCERT Exercise 3.32, page 98).
(a) Li2O; (b) Mg3N2; (c) AlI3; (d) SiO2; (e) PF3 or PF5; (f) element 71 is lutetium, a lanthanoid with valence 3: LuF3.
Q12. Which does not affect the valence shell: (a) the valence principal quantum number, (b) the nuclear charge, (c) the nuclear mass, (d) the number of core electrons (NCERT Exercise 3.35, page 99)?
(c), the nuclear mass, in the approximation used for these periodic trends. Small isotope shifts are beyond that approximation; the other factors have much larger effects.
Q13. Which statement about ionisation enthalpy is incorrect? (a) It increases for each successive electron. (b) The greatest increase comes on removing an electron from a noble gas core. (c) The end of the valence electrons is marked by a big jump. (d) Removing an electron from an orbital of lower n is easier than from one of higher n (NCERT Exercise 3.37, page 99).
(d). For the inner-core versus valence comparison intended here, the lower-shell core electron is more tightly bound and harder to remove. In general orbital energy also depends on subshell and occupancy, not n alone.
Q14. The successive ionisation enthalpies of an element are 738, 1451, 7733 and 10 543 kJ mol−1. Which group is it in?
The big jump comes after the second electron (1451 to 7733, over five times), so there are two valence electrons: group 2 (magnesium).