TRENDS IN PROPERTIES OF p-BLOCK ELEMENTS.

(A) GROUP 13 ELEMENTS : THE BORON FAMILY

Oxidation state and trends in chemical reactivity :

$ \text{General Oxidation State} =+3$.

Reactivity towards acids and alkalies

$ 2Al(s) + 6HCl(aq) \longrightarrow 2 AI^{3+} (aq) + 6 CI^-(aq) + 3H_2(g)$

$2Al(s) + 2NaOH(aq) + 6H_2O(1) \longrightarrow 2Na^+[Al(OH)_4]^-(aq)+3H_2(g)$

$\hspace{60mm} \text{Sodium tetrahydroxoaluminate(lll)} $

Reactivity towards halogens

$ 2 E(s)+3 X_2(g) \rightarrow 2 EX_3(s) \quad(X=F, Cl \hspace{1mm}Br, I) $

BORON (B):

Some Important Reactions of Boron and its compounds :

Hrdrogen Bonding in Boric Acid

PYQ-2023 p Block Elements Group 13 Q5

Back Bonding

PYQ-2023 p Block Elements Group 13 Q6

• In compounds like BF3, the boron atom has an incomplete octet. The fluorine atom on its side has a lone pair which it can donate to boron. But, flurorine is also a very electronegative element. So, it also has a tendency to take back the electrons that it had donated to boron. This way, the lone pair of electrons keep jumping between fluorine and boron. This is called back bonding. This provides the lone pair of electrons more number of exchange positions (which simply means more space). As a result, the molecule becomes more stable.

• Back bonding is effective only when the size of the valence shell matches. In the case of BF3, both boron and fluorine have their valence electrons in 2p. But in BBr3, lone pair electrons are in 4p while valence electrons of Boron are in 2p. So, the size does not match. Also, electronegativity of the halogen decreases down the group.

Reduction of Beryllium

PYQ-2023 p Block Elements Group 13 Q1

Beryllium has less negative value compared to other alkaline earth metals. However its reducing nature is due to large hydration energy associated with the small size of Be2+ ion and relatively large value of the atomization enthalpy of the metal.

• Small amines such as $NH_3, CH_3 NH_2$ and $(CH_3)_2 NH$ give unsymmetrical cleavage of diborane.

• $B_2 H_6+2 NH_3 \longrightarrow [H_2 B(NH_3)_2]^{+} + [BH_4]^{-}$

$\quad$ Large amines such as $(CH_3)_3 N$ and pyridine give symmetrical cleavage of diborane.

• $2(CH_3)_3 N+B_2 H_6 \longrightarrow 2 H_3 B \longleftarrow N(CH_3)_3$

$\quad$ $B_2 H_6 + 2 CO \xrightarrow {200^{\circ} C, 20 atm} 2 BH_3 CO \text { (borane carbonyl) }$

Pi-Bond Calculation

PYQ-2023 p Block Elements Group 14,15,16 Q3

Chemical Reaction

PYQ-2023 p Block Elements Group 14,15,16 Q1 , PYQ-2023 p Block Elements Group 14,15,16 Q2 , PYQ-2023 p Block Elements Group 14,15,16 Q4 , PYQ-2023 p Block Elements Group 14,15,16 Q5 , PYQ-2023 p Block Elements Group 14,15,16 Q6

1. 

2. Reaction of Phosphorus oxoacids with $AgNO_3$

$$\mathrm{H}_3 \mathrm{PO}_3+2 \mathrm{AgNO}_3+\mathrm{H}_2 \mathrm{O} \rightarrow \mathrm{H}_3 \mathrm{PO}_4+2 \mathrm{Ag}+2 \mathrm{HNO}_3$$

3. Reaction of thionyl Chloride with White Phosphorus

4. Reaction of $NO_2$ in presence of Sunlight

5. Reaction of Chlorine Salt

(B) GROUP 14 ELEMENTS : THE CARBON FAMILY

Carbon $(\mathrm{C})$, silicon $(\mathrm{Si})$, germanium $(\mathrm{Ge})$, tin $(\mathrm{Sn})$ and lead $(\mathrm{Pb})$ are the members of group 14.

Electronic Configuration $=\mathrm{ns}^{2} \mathrm{np}^{2}$

Oxidation states and trends in chemical reactivity

Common oxidation states $=+4$ and $+2$ . Carbon also exhibits negative oxidation states. In heavier members the tendency to show $+2$ oxidation state increases in the sequence $\mathrm{Ge}<\mathrm{Sn}<\mathrm{Pb}$

(i) Reactivity towards oxygen :

All members when heated in oxygen form oxides. There are mainly two types of oxides, i.e. monoxide and dioxide of formula $\mathrm{MO}$ and $\mathrm{MO}_{2}$ respectively.

(ii) Reactivity towards water :

Tin decomposes steam to form dioxide and dihydrogen gas.

(iii) Reactivity towards halogen :

These elements can form halides of formula $MX_2$ and $MX_4$ ( where $X=F, Cl$ $Br, I$). Stability of dihalides increases down the group.

ANOMALOUS BEHAVIOUR OF CARBON :

Catenation :

The order of catenation is $\mathrm{C}> >\mathrm{Si}>\mathrm{Ge} \approx \mathrm{Sn}$. Lead does not show catenation. Due to the property of catenation and $p \pi-p \pi$ bonds formation, carbon is able to show allotropic forms.

Bond $\hspace{10mm}$ Bond enthalpy (kJ mol-1)

PYQ-2023 p Block Elements Group 13 Q7

$C \text{\textemdash} C\hspace{15mm} 348$

$Ge \text{\textemdash} Ge\hspace{12mm} 260$

$Si \text{\textemdash} Si\hspace{13mm} 297$

$Sn \text{\textemdash} Sn\hspace{12mm} 240$

Allotropes of Carbon

Diamond :

Crystalline lattice $\mathrm{sp}^{3}$ hybridization and linked to four other carbon atoms by using hybridized orbitals in tetrahedral manner. The $\mathrm{C}-\mathrm{C}$ bond length is $154 \mathrm{pm}$. and produces a rigid three dimensional network of carbon atoms.

Graphite :

Graphite has layered structure. Layers are held by van der Waal’s forces and distance between two layers is $340 \mathrm{pm}$. Each layer is composed of planar hexagonal rings of carbon atoms. $\mathrm{C}-\mathrm{C}$ bond length within the layer is $141.5 \mathrm{pm}$. Each carbon atom in hexagonal ring undergoes $\mathrm{sp}^{2}$ hybridization graphite conducts electricity along the sheet. Graphite cleaves easily between the layers and therefore, it is very soft and slippery. For this reason graphite is used as a dry lubricant in machines running at high temperature.

Fullerenes :

$C_{60}$ molecule has a shape like soccer ball and called Buckminsterfullerene. It contains twenty six -membered rings and twelve five membered rings. This ball shaped molecule has 60 vertices and each one is occupied by one carbon atom and it also contains both single and double bonds with $\mathrm{C}-\mathrm{C}$ distance of $143.5 \mathrm{pm}$ and $138.3 \mathrm{pm}$ respectively.

SOME IMPORTANT REACTIONS OF $CO$, $CO_2$ AND METAL CARBIDES :

PYQ-2023 p Block Elements Group 13 Q2

CLASSIFICATION OF SILICATES :

(C) Cyclic Silicates :

(D) Chain Silicates :

(E) Two dimensional sheet silicates :

In such silicates, three oxygen atoms of each tetrahedral are shared with adjacent $SiO_4^{4-}$ tetrahedrals. Such sharing forms two dimension sheet structure with general formula $(Si_2 O_5)_{n}{ }^{2 n-}$

(F) Three dimensional sheet silicates :

These silicates involve all four oxygen atom in sharing with adjacent $\mathrm{SiO}_{4}^{4-}$ tetrahedral units.

SILICONES :

$\bullet \hspace{1mm}$ Silicones can be prepared from the following types of compounds only. (i) $R_3 SiCl$ (ii) $R_2 SiCl_2$ (iii) $RSiCl_3$

$\bullet \hspace{1mm}$ Silicones from the hydrolysis of $(CH_3)_3$ SiCl

$ 2(CH_3)_3 SiCl \xrightarrow{H_2 O} 2(CH_3)_3 Si(OH) \longrightarrow {}_3(H_3C)Si-O-Si(CH_3)_3$

$\bullet \hspace{1mm}$ Silicones from the hydrolysis of a mixture of $(CH_3)_3 SiCl \And (CH_3)_2 SiCl_2$

$\bullet \hspace{1mm}$ When a compound like $CH_3 SiCl_3$ undergoes hydrolysis, a complex crosslinked polymer is obtained.

$\bullet \hspace{1mm}$ The hydrocarbon layer along the silicon-oxygen chain makes silicones water-repellent.

Trend of Ionization Enthalphy

PYQ-2023 p Block Elements Group 13 Q3

• Down the group, effective nuclear charge decreases due to the addition of new shells in the atom of the elements which leads to an increased screening effect. Thus, it becomes easier to remove valence shell electrons, and hence, ionization enthalpy decreases from $\mathrm{B}$ to $\mathrm{Al}$ as expected.

• However, there is a marginal difference in the ionization enthalpy from Al to TI.

• The ionization enthalpy increases slightly for Ga but decreases from Ga to In. In the case of Ga, there are $10 \mathrm{~d}$-electrons in its inner electronic configuration which shield the nuclear charge less effectively than the $s$ and $p$-electrons and therefore, the outer electron is held fairly strongly by the nucleus. As a result, the ionization enthalpy increases slightly.

• Number of $d$ electrons and the extent of screening effect in indium is the same as that in gallium. However, the atomic size increases from Ga to In. Due to this, the first ionization enthalpy of In decreases.

• The last element $\mathrm{TI}$ has $10 \mathrm{~d}$-electrons and $14 \mathrm{f}$-electrons in its inner electronic configuration which exerts still smaller shielding effect on the outer electrons. Consequently, its first ionization enthalpy increases considerably.

Preperation of $LiALH_4$

PYQ-2023 p Block Elements Group 13 Q4

The preparation of lithium aluminum hydride (LiAlH4) involves the reaction between lithium hydride (LiH) and aluminum chloride (AlCl3).

4 LiH + AlCl3 -> LiAlH4 + 3 LiCl

This reaction is typically carried out in anhydrous conditions to prevent any moisture from interfering with the reaction. LiAlH4 is a powerful reducing agent commonly used in organic chemistry for the reduction of various functional groups.

GROUP 15 ELEMENTS : THE NITROGEN FAMILY

Electronic Configuration : $ns^2 np^3 $.

Atomic and Ionic Radii :

Covalent and ionic (in a particular state) radii increase in size down the group.

Physical Properties:

All the elements of this group are polyatomic. Metallic character increases down the group. The boiling points, in general, increase from top to bottom in the group but the melting point increases upto arsenic and then decreases upto bismuth. Except nitrogen, all the elements show allotropy.

Chemical Properties :

Oxidation States and trends in a chemical reactivity :

The common oxidation states of these elements are $-3,+3$ and +5 . The stability of +5 oxidation state decreases and that of +3 state increases (due to inert pair effect) down the group $; \mathrm{Bi}^{3+}>\mathrm{Sb}^{3+}>\mathrm{As}^{3+} ; \mathrm{Bi}^{5+}<\mathrm{Sb}^{5+}$ $<\mathrm{As}^{5+}$

Nitrogen exhibits $+1,+2,+4$ oxidation states also when it reacts with oxygen.

Anomalous properties of nitrogen :

(i) The stability of hydrides decreases from $NH_3$ to $BiH_3$ which can be observed from their bond dissociation enthalpy. Consequently, the reducing character of the hydrides increases.

Basicity also decreases in the order $NH_3 > PH_3$ $>AsH_3>SbH_3 \geq BiH_3$

PROPERTIES OF HYDRIDES OF GROUP 15 ELEMENTS

PYQ-2023 p Block Elements Group 14,15,16 Q10

(ii) The oxide in the higher oxidation state of the element is more acidic than that of lower oxidation state. Their acidic character decreases down the group. The oxides of the type $E_2 O_3$ of nitrogen and phosphorus are purely acidic, that of arsenic and antimony amphoteric and those of bismuth is predominantly basic.

(iii) Nitrogen does not form pentahalide due to non - availability of the d orbitals in its valence shell. Pentahalides are more covalent than trihalides. Halides are hydrolyzed in water forming oxyacids or oxychlorides.

$PCl_3+H_2 \longrightarrow H_3 PO_3+HCl$

$ SbCl_3+ H_2 O \longrightarrow SbOCl \downarrow \text { (orange) }+2 HCl $

$BiCl_3+ H_2 O \longrightarrow BiOCl \downarrow \text { (white) }+2 HCl$

(iv) These elements react with metals to form their binary compounds exhibiting -3 oxidation state, such as, $Ca_3 N_2$ (calcium nitride) $Ca_3 P_2$ (calcium phosphide) and $Na_3 As_2$ (sodium arsenide).

NITROGEN (N) AND ITS COMPOUNDS :

PYQ-2023 p Block Elements Group 14,15,16 Q12

$CaO+H_2 O \rightarrow Ca(OH)_2$; used for drying of $NH_3$

Oxides of Nitrogen

Blue Baby Syndrome

PYQ-2023 p Block Elements Group 14,15,16 Q7

Nitrate or excess amount of nitrate in drinking water can be hazardous to health and can cause the blue baby syndrome. This is because the greater amount of nitrate is consumed by an infant due to which oxygen cannot be carried out. The maximum amount of nitrate in drinking water is 10 parts per million.

Odd electron molecule

PYQ-2023 p Block Elements Group 14,15,16 Q9

Odd electron molecules are molecules whose total number of valence electrons is an odd number .

PHOSPHORUS (P) AND ITS COMPOUNDS :

When white phosphorus is heated in the atmosphere of $CO_2$ or coal gas at $573 K$ red phosphorus is produced. $\alpha$-black phosphorus is formed when red phosphorus is heated in a sealed tube at $803 K$. $\beta$-black phosphorus is prepared by heating white phosphorus at $473 K$ under high pressure.

Order of thermodynamic stability of various allotropes of phosphorus : black > red > white

Oxoacids of Phosphorus

PYQ-2023 p Block Elements Group 14,15,16 Q13

(D) GROUP 16 ELEMENTS : THE OXYGEN FAMILY

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

Atomic and Ionic Radii :

Due to increase in the number of shells, atomic and ionic radii increase from top to bottom in the group. The size of oxygen atoms is however, exceptionally small.

Physical Properties :

Oxygen and sulphur are non-metal, selenium and tellurium metalloids, whereas polonium is a metal. Polonium is radioactive and is short lived (Half-life 13.8 days). The melting and boiling points increase with an increase in atomic number down the group.

Catenation :

Tendency for catenation decreases down the group. This property is prominently displayed by sulphur $\left(\mathrm{S}_{8}\right)$. The $\mathrm{S}-\mathrm{S}$ bond is important in biological system and is found in some proteins and enzymes such as cysteine.

Chemical Properties

Oxidation states and trends in chemical reactivity :

Elements of the group exhibit $+2,+4,+6$ oxidation states but +4 and + 6 are more common.

Anomalous behaviour of oxygen :

The anomalous behaviour of oxygen is due to its small size and high electronegativity. The absence of $d$ orbitals in oxygen limits its covalency to four.

(i) Their acidic character increases from $H_2 O$ to $H_2$ Te. The increase in acidic character can be understood in terms of decrease in bond $(H-E)$ dissociation enthalpy down the group.

Owing to the decrease in bond $(H-E)$ dissociation enthalpy down the group, the thermal stability of hydrides also decreases from $H_2 O$ to $H_2 Po$. All the hydrides except water possess reducing property and this property increases from $H_2 S$ to $H_2$ Te.

PROPERTIES OF HYDRIDES OF GROUP 16 ELEMENTS

PYQ-2023 p Block Elements Group 14,15,16 Q11

(ii) Reducing property of dioxide decreases from $SO_2$ to $TeO_2 ; SO_2$ is reducing while $TeO_2$ is an oxidizing agent. Oxides are generally acidic in nature.

(iii) The stabilities of the halides decrease in the order $\mathrm{F}>\mathrm{Cl}>\mathrm{Br}>\mathrm{I}$. Sulphur hexafluoride $SF_6$ is exceptionally stable for steric reasons.

The well known monohalides are dimeric in nature, Examples are $S_2 F_2$, $S_2 Cl_2, S_2 Br_2, Se_2 Cl_2$ and $Se_2 Br_2$. These dimeric halides undergo disproportionation as given below :

$ 2 Se_2 Cl_2 \longrightarrow SeCl_4 +3 Se $

$\text{ OXYGEN } (O_2) \text{ AND ITS COMPOUNDS :}$

Oxo-acids of Sulphur

(E) GROUP 17 ELEMENTS : THE HALOGEN FAMILY

Fluorine, chlorine, bromine, iodine and astatine are members of Group 17.

Electronic Configuration : $\mathrm{ns}^{2} \mathrm{np}^{5}$

Atomic and Ionic Radii

The halogens have the smallest atomic radii in their respective periods due to maximum effective nuclear charge .

Physical Properties

Fluorine and chlorine are gases, bromine is a liquid whereas iodine is a solid. Their melting and boiling points steadily increase with atomic number. The $\mathrm{X}-\mathrm{X}$ bond disassociation enthalpies from chlorine onwards show the expected trend : $\mathrm{Cl}-\mathrm{Cl}>\mathrm{Br}-\mathrm{Br}>\mathrm{F}-\mathrm{F}>\mathrm{I}-\mathrm{I}$.

Chemical Properties

PYQ-2023 p Block Elements Group 14,15,16 Q14

Oxidation states and trends in chemical reactivity

PYQ-2023 p Block Elements Group 14,15,16 Q15

All the halogens exhibit -1 oxidation state. However, chlorine, bromine and iodine exhibit $+1,+3,+5$ and +7 oxidation states also.

$2F_2(g) + 2 H_2 O(\ell) \longrightarrow 4 H^+ (aq) + 4F^-(aq)+ O_2(g)$

$X_2(g) + H_2O(\ell) \longrightarrow HX(aq) + HOX(aq); \hspace{5mm} \text{(where X = Cl or Br )}$

$4I^-(aq) + 4H^+ (aq) + O_2(g) \longrightarrow 2 I_2 (s) + 2H_2 O(\ell)$

(F) GROUP 18 ELEMENTS : (THE ZERO GROUP FAMILY)

Helium, neon, argon, krypton, xenon and radon .

Most abundant element in air is Ar. Order of abundance in the air is Ar $>$ $\mathrm{Ne}>\mathrm{Kr}>\mathrm{He}>\mathrm{Xe}$.

Electronic Configuration : $\mathrm{ns}^{2} \mathrm{np}{ }^{6}$

Atomic Radii

Atomic radii increase down the group with increase in atomic number.

Physical properties:

All the noble gases are mono-atomic. They are colourless, and tasteless. They are sparingly soluble in water. They have very low melting and boiling points because the only type of interatomic interaction in these elements is weak dispersion forces.

Chemical Properties :

In general, noble gases are least reactive. Their inertness to chemical reactivity is attributed to the following reasons:

(i) The noble gases except helium $1\mathrm{~s}^{2}$ have completely filled $n s^{2} n p^{6}$ electronic configuration in their valence shell.

(ii) They have high ionization enthalpy and more positive electron gain enthalpy. The reactivity of noble gases has been investigated occasionally ever since their discovery, but all attempt to force them to react to form the compounds were unsuccessful for quite a few years. In March 1962, Neil Bartlett, then at the University of British Columbia, observed the reaction of a noble gas.

First, he prepared a red compound which is formulated as $O_2{ }^+ PtF_6{ }^- $. He , then realized that the first ionization enthalpy of molecular oxygen $ (1175 KJ mol^{-1}) $ was almost identical with that xenon $(1170 KJ mol$ ${ }^{-1})$. He made efforts to prepare same type of compound with $Xe^+ PtF_6{ }^- $ by mixing $ Pt_6$ and Xenon. After this discovery, a number of xenon compounds mainly with most electronegative elements like fluorine and oxygen, have been synthesized.

• If Helium is compressed and liquified it forms $He(I)$ liquid at $4.2 K$. This liquid is a normal liquid like any other liquid. But if it is further cooled then $He(II)$ is obtained at $2.2 K$, which is known as super fluid, because it is a liquid with properties of gases. It climbs through the walls of the container & comes out. It has very high thermal conductivity & very low viscosity.

Clathrate compounds :

During the formation of ice Xe atoms will be trapped in the cavities (or cages) formed by the water molecules in the crystal structure of ice. Compounds thus obtained are called clathrate compounds.

Clathrate provides a convenient means of storing radioactive isotopes of $\mathrm{Kr}$ and $\mathrm{Xe}$ produced in nuclear reactors.