Q4. Mendeleev predicted an atomic weight of 68 and a density of 5.9 g cm for eka-aluminium. Gallium has 70 and 5.94 g cm (NCERT Table 3.3). Find the percentage error in each prediction.
Atomic weight: (70 − 68) ÷ 70 × 100 = 2.9%. Density: (5.94 − 5.9) ÷ 5.94 × 100 = 0.67%.
Q5. Predict the formulas of the oxide and chloride of eka-silicon from its place below silicon, whose compounds are SiO2 and SiCl4.
Same group, same formula type: EO2 and ECl4, as found for germanium (GeO2, GeCl4).
Q6. Tellurium (127.6) is heavier than iodine (126.9). Where would strict atomic-weight order have put iodine, and why did Mendeleev not do so?
Strict order would put iodine before tellurium, in tellurium’s group (Group VI). Mendeleev placed iodine in Group VII with fluorine, chlorine and bromine because its properties match theirs, and suspected the atomic weights were wrong. Later, ordering by atomic number (Te 52, I 53) showed his placement was right.
Q7. Counting lithium as the first, which element in Newlands’ order Li, Be, B, C, N, O, F, Na, Mg, … is the eighth, and which is the fifteenth? What do they have in common with lithium?
The eighth is sodium and the fifteenth is potassium (Na, Mg, Al, Si, P, S, Cl, K are the 8th to 15th). Both are soft, reactive alkali metals like lithium.