(i) At room temperature, HCI is a colourless gas and the solution of HCI in water is called hydrochloric acid and it is in liquid state.
(ii) Sodium hydride NaH is an ionic compound and it is made of sodium cations (Na⁺) and hydride (H^-) anions. It has the octahedral crystal structure. It is an alkali metal hydride.

(i) Hydrogen bonds occur in complex biomolecules such as proteins and in biological systems.
(ii) For example, hydrogen bonds play an important role in the structure of deoxyribonucleic acid (DNA), since it holds together the two helical nucleic acid chains.
(iii) In these systems, hydrogen bonds are formed between specific pairs, for example, with a thymine unit in one chain bonding to an adenine unit in another; similarly, a cytosine unit in one chain bonds to a guanine unit in another.
(iv) Intramolecular hydrogen bonding also plays an important role in the structure of polymers, both synthetic and natural.

(i) Intramolecular hydrogen bonds are those which occur within one single molecule. This occurs between two functional groups within a molecule.

(ii) An intramolecular hydrogen bond (dashed lines) joins the OH group to the doubly bonded oxygen atom of the carboxyl group on the same molecule. e.g., Salicylic acid.

(iii) Salicylic acid act as an analgesic and antipyretic.

When compounds containing hydrogen are treated with D₂O, hydrogen undergoes an exchange for Deuterium.
2NaOH + D₂O ⟶ 2NaOD + HOD
Sodium hydroxide Heavy water Sodium deutroxide
HCI +  D₂O ⟶ DCI + HOD
Hydrogen chloride Deuterium Chloride
NH₄Cl + 4D₂O ⟶ ND₄Cl + 4HOD
Ammonium chloride Deutero Ammonium Chloride

The distance between the nuclei of two covalently bonded atoms is known as covalent distance or inter-nuclear distance. The one-half of this inter-nuclear distance is called covalent radius.
The covalent distance (Cl-Cl) of Cl₂ molecule is experimentally found as 198 pm (1.98 \(\mathring { A } \)). Its covalent radius is 99 pm (0.99 \(\mathring { A } \)).
Cl-Cl Inter Inter nuclear distance = 1.98 \(\mathring { A } \)
∴ r_Cl =1.98/2 = 0.99 \(\mathring { A } \)

Mg (Z=12)1s² 2s² 2p⁶ 3s².
Al (Z=13) 1s² 2s² 2p⁶ 3s² 3p¹.
Although the nuclear charge of aluminium is greater than that of magnesium, I.E of Mg is greater than that of AI. It is because Mg atom has more stable configuration than Al atom.
\(\therefore\) I.E₁ of Mg>LE₁ of AI.

X³⁺,Y²⁺, Z^- are isoelectronic.
∴ Effective nuclear charge is in the order
(Z_eff)_Z-< (Z_eff)_y2+< (Z_eff)_X3+ and hence 
ionic radius should be in the order r_z- > r_y²⁺ >r_X³⁺
∴ The correct values are

\({ O }_{ (g) }+{ e }^{ - }\longrightarrow { O }^{ - }_{ (g) };{ \Delta }_{ eg }H=-141\quad kj\quad { mol }^{ -1 }\)
\({ O }_{ (g) }+{ e }^{ - }\longrightarrow { O }^{ 2- }_{ (g) };{ \Delta }_{ eg }H=+780\quad kj\quad { mol }^{ -1 }\)
When an electron is added to oxygen atom to form ion, energy is released. Hence, first electron gain enthalpy of oxygen is negative.
But when another electron is added to O^- ion to form O^2- ion, it feels stronger electrostatic repulsion. Hence, addition of second electron takes place with absorption of energy. That's why the second electron gain enthalpy of oxygen is positive.

(i) Marine species cannot live in distilled water because it does not contain dissolved oxygen.
(ii) Distilled water is also called as deionised water, because it does not contain any cations and anions.

The element belongs to d- block, Zinc (Z=30).

Elements that show properties that are characteristic of both metals and non-metals are called as Semi Metals or Metalloids. Example- germanium, silicon, arsenic, antimony and tellurium.

1. Deuterium is used in heavy water moderated fission reactors, usually as liquid D₂O to slow neutrons
2. To prepare D₂O (heavy water)
3. To prepare deuterated halides
4. To prepare compounds like deutero methane, deutero ammonia etc..
5. To manufacture deuterium lamps with quartz or UV glass bulb.

Elements having same atomic number but different mass numbers are called isotopes. These are variant, of an element which differ in neutron numbers, which contain equal number of protons. They differ in relative atomic mass but not in chemical properties.
The isotopes of hydrogen are:
1. Protium \(\left(\mathrm{H}^{1} \text { or } \mathrm{H}\right)\) [Predominant form]
2. Deuterium (or) Heavy hydrogen \(\left({ }_{1} \mathrm{H}^{2} \text { or }{ }_{1} \mathrm{D}^{2}\right)\)
3. Tritium \(\left({ }_{1} \mathrm{H}^{3} \text { or }{ }_{1} \mathrm{T}^{3}\right)\)

The carbon monoxide of the water gas can be converted to carbon dioxide by mixing the gas mixture with more steam at 400^oC and passed over a shift converter containing iron/copper catalyst. This reaction is called as water-gas shift reaction.
CO + H₂O ➝ CO₂ + H₂

(i) 2KMnO₄ + 3H₂O₂ ➝ 2MnO₂ + 2KOH + 2H₂O + 3O₂ (Redox reaction)
(ii) CrCl₃ + 6H₂O ➝ [Cr Cl₂ (H₂O)CI. 2H₂O [Hydration reaction]
(iii) CaO + H₂O ➝ Ca(OH)₂ (Hydration reaction)

(i) Lithium and beryllium form more covalent compounds, unlike the alkali and alkali earth metals which predominantly form ionic compounds.
(ii) The elements of the second period have only four orbitals (2s & 2p) in the valence shell and have a maximum co-valence of 4, whereas the other members of the subsequent periods have more orbitals in their valence shell and shows higher valences. For example, boron forms BF₄^- and aluminium forms AIF₆^3-.

Z = 118; [₈₆Rn] 5f¹⁴ 6d¹⁰ 7s² 7p⁶
In the periodic table the element with Z = 118 is located in p-block,
Period no = 7 (as n = 7 for valence shell)
Group no. = 18 (group no = 10+ ns electrons + np electrons) (n - outer most shell)

(i) Electronic configuration is the arrangement of electrons in an atom. The outermost electron shell is often referred to as the "valence shell"determines the chemical properties.
(ii) Ionization energy and electron affinity is the amount of energy released or required in pulling out or adding an electron to a neutral atom. So both depend on electronic configuration of the element.

Ionisation energy is the amount of energy required to remove the electron from the ground state (EI) to excited state (E^∞)
E₁=-2.18 x 10^-18 J; E_∞=0
ΔE=E_∞-E₁
=0-(-2.18 x 10^-18 J)=2.18 x 10^-18 J
I.E per hydrogen atom = 2.18 x 10^-18 J
I.E per mole of H-atom =2.18 x 10^-18 J x 6.023 x 10²³
=13.13 x 10⁵ J mol^-1