Indian Navy SSR (Sailor) Chemistry — practice by topic

All three central atoms are $sp^3$ hybridised so the base bond angle is $109.5°$. Lone pairs compress that angle (the lone-pair-to-bond-pair repulsion is stronger than bond-pair-to-bond-pair). Approximate bond angles: - $\mathrm{CH_4}$ (0 lone pairs): $109.5°$ - $\mathrm{NH_3}$ (1 lone pair): $107°$ - $\mathrm{H_2O}$ (2 lone pairs): $104.5°$ More lone pairs means more compression, so the order is $\mathrm{CH_4} > \mathrm{NH_3} > \mathrm{H_2O}$.

Modern (Moseley's) periodic law: properties of elements are periodic functions of their **atomic number** ($Z$). This replaced Mendeleev's original law which was based on atomic mass. Why atomic number wins: it explains chemistry better (electronic structure follows $Z$, not mass) and resolves Mendeleev's anomalies (the Ar/K and Te/I pairs whose mass-order disagrees with chemistry-order).

For an isothermal reversible expansion, $W_\text{by gas} = +nRT\ln(V_2/V_1) = +2.303\,nRT\log(V_2/V_1)$. In the IUPAC convention $W_\text{on system} = -nRT\ln(V_2/V_1)$ is negative, but the question asks for work done *by* the gas, which is the positive of this.