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1. It is used for purifying water.

2. It is a used as a bleaching agent.

The colour of halogens is due to the reason that their molecules absorb light in the visible region. F₂ absorbs violet light hence appears pale yellow while iodine absorbs yellow and green light and hence transmists deep violet. Similarly chlorine and bromine appear greenish yellow and orange red respectively.

The general electronic configuration of halogens is np⁵, where n = 2-6. Thus, halogens need only one more e- to complete their octet and to attain the stable noble gas configuration. Also, halogens are highly electronegative with low dissociation energies and high negative electron gain enthalpies. Therefore, they have a high tendency to gain an election. Hence, they act as strong oxidizing agents.

(i) Cl₂ + H₂O → HCl + HOCl

(Hydrochloric acid) (hypochlorous acid)

(ii) 2F₂ + 2H₂O → 4H^-(aq) + 4F^-(aq) + O₂ + 4HF(aq)

Two or more forms of the same elements in same physical state which differ in their physical properties but have same chemical properties are called allotropic forms or (allotropes) and the phenomenon is called allotropy.

Carbon, phosphorus and sulphur are some elements which exhibit allotropy.

(i) Diamond and graphite are allotropic forms of carbon 

(ii) Red phosphorus and white phosphorus are allotropes of phosphorus. 

(iii) Rhombic sulphur, monoclinic sulphur and plastic sulphur are allotropic forms of sulphur.

Both NH₃ and BiH₃ have lone pair of electrons on the central atom and hence should behave as Lewis bases. But NH₃ is much more basic than BiH₃. This can be explained on the basis of electron density on the central atom. Atomic size of N (70 pm) is much smaller than that of Bi (148 pm), Therefore the electron density on N – atom is much higher than that on Bi-atom. Consequently, the tendency of N in NH₃ to donote its pair of electrons is much higher than that of Bi in BiH₃. Thus, NH₃ is much more basic than BiH₃.

When PCl₅ is heated it form PCl₃ and chlorine gas.
PCl₅ \(\overset{Heat}{\longrightarrow}\) PCl₂ + Cl₂ ↓

PCl₅ can only act as an oxidizing agent. The highest oxidation state that P can show is +5. In PCl₅ , phosphorus is in its highest oxidation state (+5). However it can decrease its oxidation state and act as an oxidizing agent.

SnH₄ > GeH₄ > SiH₄ > CH₄

As we go down the group from C to Sn surface area of hydrides increases therefore, Van der Waals’ force increases hence boiling point increases.

Allotropy: The existence of same element in two or more forms in the same physical state but having different properties called allotropy. The different forms are called allotrope.

Pb: → No allotrope exists.

Sn: → Sn⁺² Sn⁺³

Ge: → Two (Both are crystalline form)

Si: → Two (Both are crystalline form)

SiCO₂ is acidic oxide because Si is non-metal.

Tincal (or Borax) is an ore of boron. Its formula is NaB₄ Or 10H₂O

It is due to strong covalent bonds.

The heavier elements of p-block elements forms π – bonds because of the combined effect of size and availability of d-orbital’s considerably influences the ability of there elements to form π – bonds.

ClO^- is isoelectronic to ClF. Also, both species contain 26e^-s in all as shown.

Total electrons in ClO^- = 17 + 8 + 1 = 26

In ClF = 17 + 9 = 26

ClF, acts like a Lewis base as it accepts e^-s from F to form ClF₃

BF₃ is reacted with ammonia BF₃ 

(a Lewis acid) reacts with NH₃ (a Lewis base) to form a product. This results in a complete octet around B in BF₃.

F₃B+:NH₃→F₃B←:NH₃

(i) XeO₃ can be prepared in two ways as shown:

6XeF₄ + 12H₂O → 4Xe + 2XeO₃ + 24HF + 3O₂ XeF₆ + 3H₂O → XeO₃ + 6HF

(ii) XeF₄ can be prepared using XeF₆

XeF₆+ H₂O → XeOF₄ + 2HF

(i) Bond dissociation energy usually decreases on moving down a group as the atomic size increases. However, the bond dissociation energy of F₂ is lower than that of Cl₂ and Br₂. This is due to the small atomic size of fluorine, Thus, the increasing order for bond dissociation energy among halogens is as follows:

I₂ < F₂ < Br₂ < Cl₂ 

(ii) HF < HCl < HBr < HI

The bond dissociation energy of HX molecules where X – F, Cl, Br, I, decreases with an increase in the atomic size. Since H – I bond is the weakest, HI is the strongest acid.

(iii) BiH₃ < SbH₃ < AsH₃ < PH₃ < NH₃ 

On moving from nitrogen to bismuth, the size of the atom increases while the electron density on the atom decreases. Thus, the basic strength decreases.

2RCl + Si → R₂SiCl₂

The chemical formula of Hypo phosphorus acid is H₃PO₂.

Boric acid is a weak monobasic acid which behaves as a Lewis acid. So, it is not a protic acid.

B(OH)₃ + 2HOH→B(OH)₄+⁻+H₃O⁺

It behaves as an acid by accepting a pair of electrons from ^–OH ion.

(i) It is because Pb²⁺ is more stable than Pb⁴⁺ due to inert pair effect whereas Sn⁴⁺ is more stable than Sn²⁺. Therefore Sn²⁺ is good reducing agent

3Ga⁺ → 2Ga + Ga³⁺ 

(ii) It is because Ga³⁺ is more stable than Ga⁺

On heating orthoboric acid at 370 K or above, it changes to metaboric acid and On further heating, this yields boric oxide B₂O₃.

H₃BO₃→HBO₂→B₂O₃

1. Borax:- Na₂B₄O₇10H₂O 

2. Kemite:- Na₂B₄O₇2H₂O

Boric acid is formed 

(Na₂B₃O₇ + 2HCl + 5H₂)O → 2NaCl + 4H₃BO₃

Boric acid is added to water

When boric acid is added to water, it accepts electrons from ^–OH ion.

B(OH)₃+2HOH→[B(OH)₄]⁻+H₃O⁺

The occurrence of oxidation states two units less than the group oxidation states is known as inert pair effect. (Or) Due to poor shielding effect of inner d and f-orbitals, the increased effective nuclear charge holds ns electrons tightly and thereby restricting their participation in bonding. 

Because, boron resembles both with metals and non-metals, therefore boron is metalloid.

Boron resembles both with metals and non-metals therefore, it is metalloid

Borax is heated strongly 

When heated, borax undergoes various transitions. It first loses water molecules and swells. Then, it turns into a transparent liquid, solidifying to form a glass-like material called borax bead.

NeF₂ is does not exist.

(a) Inert pair effect

As one moves down the group, the tendency of s-block electrons to participate in chemical bonding decreases. This effect is known as inert pair effect. In the case of group 13 elements, the electronic configuration is ns² np¹ and their group valence is +3. However, on moving down the group, the +1 oxidation state becomes more stable. This happens because of the poor shielding of the ns² electrons by the d- and f- electrons. As a result of the poor shielding, the ns² electrons are held tightly by the nucleus and so, they cannot participate in chemical bonding.

(b) Allotropy

Allotropy is the existence of an element in more than one form, having the same chemical properties but different physical properties. The various forms of an element are called allotropes. For example, carbon exists in three allotropic forms: diamond, graphite, and fullerenes.

(c) Catenation

The atoms of some elements (such as carbon) can link with one another through strong covalent bonds to form long chains or branches. This property is known as catenation. It is most common in carbon and quite significant in Si and S.

Due to very strong crystalline lattice, boron has unusually high melting point.

Germanium is a metalloid found in group – 14.

CO is heated with ZnO

When CO reacts with ZnO, it reduces ZnO to Zn. CO acts as a reducing agent.

ZnO_(s)+CO_(g)→Zn_(s)+CO_2(g)

It is interesting to note that the non-metals and metalloids exist only in the p-block of the periodic labels.

Although Ga has one shell more than Al, its size is lesser than Al. This is because of the poor shielding effect of the 3d-electrons. The shielding effect of d-electrons is very poor and the effective nuclear charge experienced by the valence electrons in gallium is much more than it is in the case of Al.

Boron in its halides has only six electrons in its valence shell. Therefore, it can accept a pair of electrons from any electron-rich molecule. Therefore, it acts as an electron – acceptor and called Lewis acid.

The electronic configuration is ns²np².

The elements B, Al, Ca, In and Tl are placed in the same group of the periodic table because each one has the same number of elections (ns² np¹) in its valance shell.

(i) Electronic configuration: All the elements in group 15 have 5 valence electrons. Their general electronic configuration is ns²np³ 

(ii) Oxidation states: All these elements have 5 valence electrons and require three more electrons to complete octets. However gaining electrons is very difficult as the nucleus will have to attract three more electrons. This can take place only with nitrogen as it is the smallest in size and the distance between the nucleus and the valance shell is relatively small. The remaining elements of this group show a formal oxidation state of -3 in their covalent compounds.

In addition to -3 state, N and P also show -1 and.-2 oxidation states. All the elements present in this group show +3 and +5 oxidation states. However stability of +5 oxidation state decreases down the group, whereas the stability of+3 oxidation state increases. This happens because of the inert pair effect.

(iii) Ionization energy and electronegativity: First ionization energy decreases on moving down a group. This is because of increasing atomic sizes. As we move down a group, electronegativity decreases, owing to an increase in size.

(iv) Atomic size: On moving down a group, the atomic size increases. This increase in the atomic size is attributed to an increase in number of shell.

Silicon is heated with methyl chloride at high temperature in the presence of copper

When silicon reacts with methyl chloride in the presence of copper (catalyst) and at a temperature of about 537 K, a class of organosilicon polymers called methyl substituted chlorosilane MeSiCl₃, Me₂SiCl₂, Me₃SiCl, and Me₄Si) are formed.

Electronic configuration:

ns²np^{0 - 5}

Group 13 elements are called p-block elements because the last electron is present

in the p-orbital (np₁). The valence shell configurations are B (2s² 2p¹), Al (3s², 3p¹),Ca (4s², 4p¹), In (5s² 5p¹) Tl (6s² 6p¹)

Hydrated alumina is treated with aqueous NaOH solution

When hydrated alumina is added to sodium hydroxide, the former dissolves in the latter because of the formation of sodium meta-aluminate.

Al₂O₃.2H₂O+2NaOH→2NaAlO₂+3H₂O

There are six groups of p-block elements in the periodic table numbering from 13 to 18.

\(\frac{10}{5}\)B and \(\frac{11}{5}\)B are isotopes of boron.

Boron does not form B³⁺ ion due to smallest atomic size and highest ionization energy.

Graphite has sp² hybridized carbon with a layer structure due to wide separation and weak inter – layer bonds the two adjacent layers can easily slide over each other. This makes graphite act as a lubricant.

Graphite forms hexagonal ring and undergoes sp² hybridization. The electrons are delocalized over the whole sheet. Electrons are mobile and therefore graphite conducts electricity over the sheet.