It is due to inert pair effect.

It is due to decrease in bond dissociation energy which is due to increase in atomic size.
C-C > Si-Si > Ge-Ge > Sn-Sn > Pb-Pb

Sp³

Producer gas is a mixture of CO and N₂ in the ratio of 2 : 1.

The molecules of CO2 are held together by weak van der Waals forces of attraction which can be easily overcome by collisions of the molecules at room temperature.
Consequently CO₂ is a gas.
On the other hand silicon atom forms four single covalent bonds with O-atom which are tetrahedrally arranged and form a three-dimensional structure. Thus Si0₂ is a high melting solid.

2NaBH₄ + I₂\(\rightarrow\) B₂H₆ + 2Nal + H₂.

Boron halides are lewis acids and can accept a pair of electrons from amines to form addition product.

Catenation: The property to form chains or rings not only with single bonds but also with multiple bonds with itself is called catenation.
For example, carbon forms chains with (C - C) single bonds and also with multiple bonds (C = C or C \(\equiv \) C).

Allotropy: It is the property of the element by which an element can exist in two or more forms which have same chemical properties but different physical properties due to their structures.

CO₂ is a greenhouse gas. About 75% of the solar energy reaching the earth is absorbed by the earth's surface. The rest of the heat radiates back to the atmosphere. But the heat radiated by the heated surface cannot pass freely into the space because excessive CO₂ in the atmosphere absorb more heat.
This results in increase in the average temperature of the atmosphere. This is known as global warming.

When carbon dioxide is passed through lime water it turn milky due to the formation of CaCO₃.
But when carbon dioxide is passed continuously, the milkiness disappears due to the formation of soluble Ca(HCO₃)₂.
Ca(OH)₂ + CO₂\(\longrightarrow \)CaCO₃ + H₂O
Milkiness
CO₂/H₂O \(\longrightarrow \)Ca(HCO₃)₂
Soluble in water

Al reacts with conc. HNO₃ to form a protective layer of aluminium oxide on its surface which prevents it from further reaction.
\(2Al(s)+6HNO_{ 3 }(conc.)\rightarrow Al_{ 2 }O_{ 3 }(s)+6NO_{ 2 }(g)+3H_{ 2 }O(l)\)
                                         Alumina (a passive protective layer)
Therefore, Al becomes passive that's why aluminium containers can be used to transport conc.HNO₃.

Being a reducing agent, tin(II) chloride reduces mercuric chloride (HgCl₂) to mercurous chloride (HgCl₂) and ferric salts to ferrous salts.
SnCl₂ + 2HgCl₂\(\longrightarrow \) SnCl₄ + Hg₂Cl₂
SnCl₂ + 2FeCl₃\(\longrightarrow \) SnCl₄ + 2FeCl₂

BF₃ achieve it by the following ways
(i) Multiple bonding or \(p\pi -p\pi \) back bonding e.g BF₃ in which a lone pair of electron present in 2p-orbits of one of the fluorine atoms may be transferred to the vacant p-orbital on the bottom atom.
(ii) Formation of complexes in which electrons are received from a donor molecule,e.g F₃B\(\longleftarrow \)NH₃ .Boron compounds, thus behave as Lewis acids.

Boron and aliminium show an oxidation state of +3 only because they do not exhibit insert pair effect due to the absence of d or f-electrons. Elements from Ga to TI show two oxidation states, i.e +1 and +3.The tendency to show +1 oxidation state increases down the group due to the inability of ns² electrons of valence shell to participate in bonding which is called inert pair effect. Therefore TI⁺ is more stable than TI³⁺