Detailed Study Notes: The p-Block Elements

General Characteristics of p-Block Elements

• Groups: Includes groups $13$ to $18$ of the periodic table.

• Electronic Configuration: General valence shell configuration is $ns^2 np^{1-6}$ (except $He$: $1s^2$).

• Influencing Factors: Atomic size, ionisation enthalpy, electron gain enthalpy, and electronegativity determine properties.

• Heavier Elements: Starting from the third period, presence of $d$ and/or $f$ orbitals significantly affects properties compared to the second period.

Group 15 Elements (The Nitrogen Family)

• Members: Nitrogen ($N$), Phosphorus ($P$), Arsenic ($As$), Antimony ($Sb$), and Bismuth ($Bi$).

• Occurrence:     * $N_2$: $78\%$ of atmosphere by volume.     * Chile saltpetre: $NaNO_3$. Indian saltpetre: $KNO_3$.     * Phosphorus: Found in apatite family minerals like fluorapatite ($Ca_9(PO_4)_6 \cdot CaF_2$).

• Electronic Configuration: $ns^2 np^3$ (half-filled $p$ orbitals provide extra stability).

• Trends:     * Atomic Radii: Increases down the group.     * Ionisation Enthalpy: Decreases down the group; higher than Group $14$ due to stable half-filled configuration.     * Electronegativity: Decreases down the group.

• Physical Properties: Metallic character increases down the group ($N, P$ non-metals; $As, Sb$ metalloids; $Bi$ metal).

• Oxidation States: Common states are $-3, +3, +5$. Stability of $+5$ decreases and $+3$ increases (Inert pair effect) down the group.

• Anomalous Nitrogen: Small size, high E.N., and lack of $d$ orbitals lead to $p\pi-p\pi$ multiple bonding ($N \equiv N$) and restricted covalency of $4$.

Nitrogen and Phosphorus Compounds

• Dinitrogen ($N_2$):     * Preparation: Commercial liquefaction of air; Lab: $NH_4Cl(aq) + NaNO_2(aq) \rightarrow N_2(g) + 2H_2O(l) + NaCl(aq)$.     * Properties: Chemically inert at room temperature due to high bond enthalpy ($941.4\,kJ\,mol^{-1}$).

• Ammonia ($NH_3$):     * Manufacture: Haber’s process: $N_2(g) + 3H_2(g) \rightleftharpoons 2NH_3(g)$. Optimum conditions: $200 \times 10^5\,Pa$, $700\,K$, iron oxide catalyst with $K_2O$ and $Al_2O_3$.     * Properties: Trigonal pyramidal; acts as a Lewis base due to lone pair on $N$.

• Nitric Acid ($HNO_3$):     * Manufacture: Ostwald’s process (Catalytic oxidation of $NH_3$).     * Properties: Strong oxidising agent; attacks most metals.

• Phosphorus allotropes: White (reactive, tetrahedral $P_4$), Red (polymeric, stable), Black ($\alpha$ and $\beta$ forms).

• Phosphine ($PH_3$): Colourless gas, rotten fish smell, highly poisonous.

• Phosphorus Halides: $PCl_3$ (pyramidal) and $PCl_5$ (trigonal bipyramidal in gas; ionic solid $[PCl_4]^+ [PCl_6]^-$).

Group 16 Elements (The Oxygen Family)

• Members: Oxygen ($O$), Sulphur ($S$), Selenium ($Se$), Tellurium ($Te$), and Polonium ($Po$).

• Also known as: Chalcogens (ore-forming elements).

• Electronic Configuration: $ns^2 np^4$.

• Oxidation States: $-2$ (most common for $O$), $+2, +4, +6$. Stability of $+6$ decreases down, $+4$ increases (inert pair effect).

• Dioxygen ($O_2$): Paramagnetic; prepared by thermal decomposition of chlorates ($KClO_3$) or oxides ($Ag_2O, HgO$).

• Ozone ($O_3$): Allotropic form of oxygen; powerful oxidising agent ($O_3 \rightarrow O_2 + O$); angular structure ($117^\circ$).

• Sulphur: Yellow rhombic ($\alpha$-sulphur) and Monoclinic ($\beta$-sulphur). Transition temperature is $369\,K$.

• Sulphuric Acid ($H_2SO_4$): Manufactured by Contact Process (Catalyst: $V_2O_5$). Strong dibasic acid and dehydrating agent.

Group 17 Elements (The Halogens)

• Members: Fluorine ($F$), Chlorine ($Cl$), Bromine ($Br$), Iodine ($I$), and Astatine ($At$).

• Electronic Configuration: $ns^2 np^5$.

• Trends: Smallest atomic radii in periods; highest negative electron gain enthalpy. $F$ is the most electronegative.

• Oxidising Power: F_2 > Cl_2 > Br_2 > I_2. $F_2$ is the strongest oxidising halogen.

• Chlorine ($Cl_2$): Greenish-yellow gas; prepared by Deacon’s process ($HCl$ oxidation over $CuCl_2$) or electrolysis of brine.

• Interhalogens: Compounds of type $XX'_n$ ($n=1,3,5,7$); more reactive than parent halogens because $X-X'$ bond is weaker than $X-X$.

Group 18 Elements (The Noble Gases)

• Members: Helium ($He$), Neon ($Ne$), Argon ($Ar$), Krypton ($Kr$), Xenon ($Xe$), and Radon ($Rn$).

• Electronic Configuration: $ns^2 np^6$ ($He$: $1s^2$).

• Chemical Properties: Very low reactivity due to stable closed-shell configuration.

• Xenon Compounds: Neil Bartlett prepared first compound $Xe^+PtF_6^-$ in $1962$. $Xe$ forms fluorides ($XeF_2, XeF_4, XeF_6$) and oxides ($XeO_3, XeOF_4$).

• Helium: Lowest boiling point ($4.2\,K$); used in diving apparatus and as a cryogenic agent.

Questions & Discussion

• Q: Why does $NO_2$ dimerise?     * A: $NO_2$ contains an odd number of valence electrons. It dimerises to form stable $N_2O_4$ with an even number of electrons.

• Q: Why is $NH_3$ a Lewis base?     * A: The nitrogen atom in $NH_3$ has a lone pair of electrons available for donation.

• Q: Why is $H_2O$ a liquid and $H_2S$ a gas?     * A: Due to high electronegativity of oxygen, $H_2O$ molecules are associated through strong hydrogen bonding.

• Q: Reactivity of $N_2$ at room temperature?     * A: Low reactivity due to extremely high bond dissociation enthalpy of the $N \equiv N$ triple bond.

• Q: Why does $PCl_3$ fume in moisture?     * A: It hydrolyses to give fumes of $HCl$: $PCl_3 + 3H_2O \rightarrow H_3PO_3 + 3HCl$.

• Q: Covalence of nitrogen in $N_2O_5$?     * A: The covalence is $4$.

• Q: What is the acidic strength order of hydrogen halides?     * A: HF < HCl < HBr < HI.

• Q: Why are halogens coloured?     * A: Due to absorption of radiation in the visible region, which excites outer electrons to higher energy levels.

• Q: Basicity of $H_3PO_4$?     * A: It is tribasic (contains three $-OH$ groups).

• Q: Why are noble gases known as noble gases?     * A: Their valence shells are completely filled, and they react with very few elements under specific conditions.

• Q: Why is $PCl_5$ prepared by SO2Cl2 action on Phosphorus?     * A: $P_4 + 10SO_2Cl_2 \rightarrow 4PCl_5 + 10SO_2$.

• Q: Condition to maximise ammonia yield in Haber's process?     * A: High pressure ($200\,atm$) as per Le Chatelier’s principle.