NEET The s-Block Elements Previous Year Questions with Solutions

According to NCERT s-Block Elements (Biological Importance of Magnesium and Calcium): "The daily requirement in the human body has been estimated to be 200 – 300 mg (0.2 – 0.3 g)."

Calcium ions (Ca²⁺) play critical roles in neuromuscular function, interneuronal transmission, cell membrane integrity, and blood coagulation. Magnesium (Mg²⁺) is not the primary ion for interneuronal transmission:

All enzymes that utilize ATP in phosphate transfer require magnesium (Mg²⁺) as the cofactor, forming an essential [Mg-ATP]²⁻ complex. Calcium is not the cofactor for these kinases:

Bone is not an inert and unchanging substance; it is constantly being dissolved, redeposited, and dynamically remodeled through osteoblast and osteoclast activity:

Alkali metals dissolve in liquid ammonia to form dilute solutions that are conducting and deep blue in color:

The ammoniated electrons absorb energy in the red region of visible light, imparting a deep blue color. Because these solvated electrons possess unpaired spins, the solution is paramagnetic. Assertion A is TRUE:

Reason R asserts that the blue color is due to the formation of sodamide (NaNH₂). In reality, amide formation destroys the ammoniated electrons and liberates H₂ gas, bleaching the blue color:

Beryllium has only 4 valence electrons in monomeric $BeCl_2$, making it severely electron deficient. In the solid state, $BeCl_2$ forms an infinite polymeric chain structure. Each beryllium atom is tetrahedrally surrounded by four chlorine atoms via two normal covalent bonds and two coordinate bonds from chlorine lone pairs ($sp^3$ hybridized Be):

When heated into the vapour phase, the polymeric chain breaks down into volatile chloro-bridged dimers, $(BeCl_2)_2$ or $Be_2Cl_4$, where each beryllium is $sp^2$ hybridized with a vacant $p$ orbital. (Only at extreme temperatures exceeding $1200\text{ K}$ does the dimer dissociate into a linear monomer $Cl-Be-Cl$ with $sp$ hybridization):

In the solid state and vapour phase, the structures are chain and dimer respectively, corresponding to Option (1).

According to Fajan's rules, covalent character in an ionic compound increases with: 1. Small size of the cation 2. High positive charge on the cation 3. High charge-to-size ratio (polarizing power)

Beryllium ($Be^{2+}$) has an exceptionally small ionic radius (approximately $31\text{ pm}$), giving it an extraordinarily high polarizing power compared to $Mg^{2+}, Ca^{2+}, Sr^{2+},$ and $Ba^{2+}$. It strongly polarizes the electron clouds of adjacent chloride anions, pulling electron density into the internuclear region and imparting strong covalent character.

Because $BeCl_2$ is essentially covalent and non-ionic, it does not form an expansive ionic crystal lattice that requires polar water for hydration. Instead, following the "like dissolves like" principle, it dissolves readily in non-polar and moderately polar organic solvents such as ether, benzene, and alcohols, whereas chlorides of $Mg, Ca,$ and $Sr$ are ionic and insoluble in non-polar organic solvents.

Crude sodium chloride obtained by evaporation of seawater contains impurities such as sodium sulphate ($$\text{Na}_2\text{SO}_4$$), calcium sulphate ($$\text{CaSO}_4$$), calcium chloride ($$\text{CaCl}_2$$), and magnesium chloride ($$\text{MgCl}_2$$). The chlorides of calcium and magnesium are deliquescent (they absorb moisture from the air).

To obtain pure $$\text{NaCl}$$, crude salt is dissolved in a minimum amount of water and filtered to remove insoluble impurities. Hydrogen chloride gas ($$\text{HCl}$$) is then passed through the saturated solution. The complete ionization of $$\text{HCl}$$ provides a high concentration of chloride ions ($$\text{Cl}^-$$).

Because $$\text{NaCl}$$ has a substantially lower solubility product than the extremely soluble $$\text{MgCl}_2$$ and $$\text{CaCl}_2$$, the common ion effect causes the ionic product $$[\text{Na}^+][\text{Cl}^-]$$ to exceed the solubility product ($$K_{sp}$$) of $$\text{NaCl}$$: $$[\text{Na}^+][\text{Cl}^-] > K_{sp}(\text{NaCl})$$ Pure sodium chloride crystallises out.

Calcium and magnesium chlorides remain in the mother liquor because they are vastly more soluble than sodium chloride and their ionic products do not exceed their large $$K_{sp}$$ values under these conditions. Thus, ONLY NaCl crystallises.

In the s-block elements chapter, NCERT presents an explicit section detailing the biological roles of sodium and potassium ions in human physiology.

NCERT states verbatim: "Potassium ions are the most abundant cations within cell fluids, where they activate many enzymes, participate in the oxidation of glucose to produce ATP and, with sodium, are responsible for the transmission of nerve signals."

The operating concentration gradient across the neuronal membrane (high $$K^+$$ inside, high $$Na^+$$ outside) is maintained by the active $$Na^+/K^+$$ pump, which consumes more than one-third of total resting ATP in a resting animal to propagate nerve impulses.

- Calcium ($$Ca^{2+}$$): Essential for neuromuscular excitability, blood clotting, and bone mineralization. - Iron ($$Fe^{2+}/Fe^{3+}$$): Part of haemoglobin and cytochromes for electron transport. - Copper ($$Cu^{2+}$$): Component of cytochrome c oxidase and tyrosinase. Thus, Potassium is uniquely described by all three statements.