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The halides of the elements having vacant d-orbital’s can form complexes like [SiF₆]^2- and [SnCl₆]^2- ,because in such a case the central atom can increase its coordination number from 4 to 6 due to availability of vacant d–orbital’s.

The B–F bond length in BF₃ is shorter than the B–F bond length in BF₄^– . BF₃ is an electron deficient species. With a vacant p-orbital on boron, the fluorine and boron atoms undergo pn–pn back-bonding to remove this deficiency. This imparts a double bond character to the B–F bond.

This double-bond character causes the bond length to shorten in BF₃ (130 pm). However, when BF₃ coordinates with the fluoride ion, a change in hybridization from sp² (in BF³) to sp³ (in BF₄^–) occurs. Boron now forms 4σ bonds and the double-bond character is lost. This accounts for a B–F bond length of 143 pm in BF₄^– ion.

Inert pair is more prominent as we move down the group in p – block elements. Ge, Sn and Pb show divalency due to inert pair effect.

The electronic configuration of carbon atom is 1s² 2s² 2px¹ 2py¹ and has four valence electrons. In order to form ionic compound, it has to either lose four electrons or gain four electrons. Since very high energy are involved in doing so. Carbon does not form ionic compounds. It completes its octet by sharing of electrons and forms covalent compounds.

Oxygen is smaller in size as compared to sulphur. Due to its smaller size, it can effectively form pπ-pπ bonds and form O₂ (O = O) molecule. Also, the intermolecular forces in oxygen are weak Van dar Wall’s which cause it to exist as gas. On the other hand, Sulphur does not form M₂ molecule but exists as a puckered structure held to gather by strong covalent bonds. Hence it is a solid.

Stability of an ionic compound depends on its lattice energy. More the lattice energy of a compound, more stable it will be Lattice energy is directly proportional to the charge carried by an ion. When a metal combines with oxygen, the lattice energy of the oxide involving O^2- ion is much more than the oxide involving O^- ion. Hence, we can say that formation of O^2- is energetically more favourable than formation of O^-.

An aqueous solution of ammonium chloride is treated with sodium nitrite 

NH₄Cl + NaNO₂(aq) → N₂(g) + 2H₂O(e) + NaCl(aq)

NO and HNO₃ are produced in small amounts. These are impurities that can be removed on passing nitrogen gas through aqueous H₂SO₄ containing potassium dichromate.

Due to the formation of a passive film of oxide on the surface.

Fullerenes are made by the heating of graphite in an electric arc in the presence of inert gases such as helium or argon.

Diamond is the hardest substance on the earth because it is very difficult to break extended covalent bonding.

Electronic configurations of carbon atom in the ground state is 1s²2s²2p₂¹2p₂¹2p₂⁰ During the formation of a compound the activation energy of the reactants excites the electrons to give the configuration as Then four orbitals each containing an impaired electron, mix together and form four hybridized sp³ orbitals directed from the carbon atom tetrahedrally. Hence carbon exhibits tetravalency.

As the temperature increases, the number of covalent bonds ruptured in an intrinsic semiconductor increases. This results in the increase of the electron hole pairs. As a result, the conductivity of the intrinsic semiconductor increases with rise in temperature. When electron field is applied across these materials electron migrate in one direction and the positive holes migrate is the opposite direction and thus the material conducts. At a given temperature the conductivity of an intrinsic semiconductor is constant.

Conductivity of a semiconductor can be increased by adding either pentavalent or trivalent elements. The elements added to intrinsic semiconductors to increase conductivity are called dopants, the process of addition of dopants (impurities) is called doping. The dopped semiconductors are called extrensic Semiconductors.

O = C = O, It has linear structure.

Ionic compounds

Carbon has smallest size, highest ionization energy and high electro negativity, therefore, it differs from rest of the elements.

Boron has smaller atomic size, therefore, it cannot lose three electrons because its third ionization energy is very high.

Graphite is used as the lubricant.

Graphite has a layered structure and different layers of graphite are bonded to each other by weak van der Waals‘ forces. These layers can slide over each other. Graphite is soft and slippery. Therefore, graphite can be used as a lubricant.

Silicones are rubber like polymers.

Bonor can absorb neutrons.

Both have similar charge over radius ratio, i.e., similar polarizing power.

It is because ‘B’ does not have vacant d-orbitals.

Boron forms covalent bonds because it cannot lose electrons or gain electron easily.

(i) 4NaCl + MnO₂ + 4H₂SO₄ → MnCl₂ + 4NaHSO₄ + 2H₂O + Cl₂

(ii) Cl₂ + Nal → 2NaCl + I₂

p-type semiconductor

(i) 2NaBH₄ + I₂ → 2NaI + H₂ + B₂H₆ (Diborane) 

(ii) B₂H₆ + 2NaH → 2NaBH₄ (sodium borohydride) 

(iii) 2BF₃ + 6LiH → 6LiF + B₂H₆ (Diborane) 

(iv) SiCl₄ + 2H₂O → 4HCl + Si(OH)₄ (Silicicacid)

Be and Al resemble with each other due to same charge over radius ratio.

(i) Both B and Al amphoteric oxides 

(ii) Be and Al form covalent compounds 

(iii) BeCl₂ exists as polymer whereas AlCl₃ exists as dimmer 

(iv) Both Be and Al react with NaOH to form similar compounds.

SiF₄ + 2HF → H₂SiF₆(Fluorosilicicacid)

n- type semiconductor.

(i) Carbon shows property of catenation to more extent than silicon due to small size and tendency to form pπ-pπ multiple bonds with itself 

(ii) Stability of hybrids: CH₄ is more stable than SiH₄ due to small size of ‘C’. 

(iii) CO₄ is gas whereas SiO₂ is three dimensional covalent solid, therefore highly stable as compared to CO₂.

Silicon has vacant d-orbitals, therefore, it can have coordination number more than four but carbon cannot have because it does not have vacant d-orbitals.

The hybrid orbital of silicon is sp³d².