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Limitations of valence bond theory :

• Valence Bond theory explains only the formation of covalent bond in which a shared pair of electrons comes from two bonding atoms. However, it offers no explanation for the formation of a coordinate covalent bond in which both the electrons are contributed by one of the bonded atoms.
• Oxygen molecule is expected to be diamagnetic according to this theory. The two atoms in oxygen molecule should have completely filled electronic shells which give no unpaired electrons to the molecule making it diamagnetic. However, experimentally the molecule is found to be paramagnetic having two unpaired electrons. Thus, this theory fails to explain paramagnetism of oxygen molecule.
• Valence bond theory does not explain the bonding in electron deficient molecules like B₂H₆ in which the central atom possesses a smaller number of electrons than required for an octet of electron.

i – b 

ii – a 

iii – d 

iv – c

Valence Bond theory introduced five new concepts in chemical bonding :

• Delocalization of electron over the two nuclei
• Shielding effect of electrons
• Covalent character of bond
• Partial ionic character of a covalent bond
• The concept of resonance and connection between resonance energy and molecular stability

An atom of hydrogen has one proton in its nucleus and one electron in its valence shell. It donates its valence electron, the residual ion consists of a single proton. It is on account of this fact, that hydrogen ion is called proton. 

When a molecule of hydrogen is formed from the atoms, energy is released (104 Kcal/mol). Thus the molecules possessing lower energy are more stable than the atoms.

When a molecule of hydrogen is formed from the atoms, energy is released (10⁴ kcal/ mol). The molecules possessing lower energy are more stable, than the atoms. Hence molecule of hydrogen is more stable than uncombined atoms.

(a) A reaction in which oxidation and reduction occur simultaneously is called an oxidation-reduction, or simply, a redox reaction. Redox reactions involve the transfer of electrons between two chemical species. The reaction in which electron is gained is called a reduction reaction and the reaction in which electron is lost is called oxidation reaction. The compound that loses an electron is said to be oxidized, the one that gains an electron is said to be reduced.

(b)(i) Oxidation :Zn → Zn ²⁺

Reduction : Pb²⁺ Pb

(ii) Oxidation : Zn → Zn ²⁺

Reduction : Cu²⁺ Cu

(iii) Oxidation : Br^- → Br

Reduction : Cl₂ → 2Cl^-

(iv) Oxidation : Sn^₂+ → Sn⁴⁺

Reduction : 2Hg²⁺ → Hg_²

(v) Oxidation : Cu⁺ → Cu ²⁺

Reduction : Cu⁺ → Cu

(c)(i)  Potassium undergoes oxidation as it loses an electron and forms a cation.

(ii)  Chlorine undergoes reduction as it gains an electron and forms chloride anion.

(iii)  Potassium acts a reducing agent and gets oxidised.

(iv)  Chlorine acts an oxidizing agent and gets reduced.

The bond formed between two atoms by sharing a pair of electrons, provided entirely by one of the combining atoms but shared by both is called a coordinate bond. It is represented by an arrow starting from the donor atoms and ending in the acceptor atom.

Conditions:

1. One of the two atoms must have at least one lone pair of electrons.

2. Another atom should be short of at least a lone pair of electrons.

The twolone pair of electrons in the oxygen atom of water is used to form coordinate bond with the hydrogen ion which is short of an electron resulting in the formation of the hydronium ion.H₂O + H⁺---> H₃O⁺ Over here the hydrogen ion accepts one lone pair of electrons of the oxygen atom of water molecule leading to the formation of a coordinate covalent bond.

D) both A and C

D) both B and C

 Correct option is  A) 1, 2, 3

 Correct option is  B) 3, 2,1

 Correct option is  C) X Y₂

(b) O₂ . 

It’s bond order is 2 whereas in others bond order is 1.

1. Covalent bonding is observed in atoms which are similar. Hence, covalent bonding is present in the particles which make up element L.

2. When L is heated with iron metal, it forms a compound FeL.

Here, oxidation of Fe and reduction of L occur as follows:

Fe → Fe²⁺ + 2^e– (Direct combination or synthesis)

Single covalent bond:

A covalent bond formed by the mutual sharing of one pair of electrons is called a single covalent bond, or simply a single bond. A single covalent bond is represented by a small line (−) between the two atoms.

Double covalent bond:

A covalent bond formed by the mutual sharing of two pair of electrons is called a double covalent bond, or simply a double bond. A double covalent bond is represented by two small horizontal lines (=) between the two atoms. 

E.g. O=O, O=C=O etc.

C) a →ii, b →i, c → iii

D) all of these

D) Both A and C

Hybridization is the concept of mixing atomic orbitals into new hybrid orbitals (with different energies, shapes, etc., than the component atomic orbitals) suitable for the pairing of electrons to form chemical bonds in valence bond theory.

In sp³ hybridisation,

1s-oribital and 3p orbitals are mixed to form 4 equivalent sp³ orbitals.

So every sp³ orbital contains 1/4th of s-orbital and 3/4th of p-orbitals.

D) p-p > s-p > s-s

i. Electronic configuration of O₂⁺ can be given as :

(σ1s)² (σ*1s)² (σ2s)² (σ*2s)² (σ2p_z)² (π2p_x)² (π2py)² (π*2p_x)¹ (π*2p_y)⁰

∴ Bond order = \(\frac{1}{2}\) (10 – 5) = 2.5

ii. Stability of the molecule or species ∝ Bond order

Bond order decreases as :

O₂⁺ > O₂ > O₂^- > \(O_2^{2-}\) 

∴ Stability decreases as :

O₂⁺ > O₂ > O₂^- > \(O_2^{2-}\) 

Hence, 

Least stable species is \(O_2^{2-}\).

1. If a covalent bound is formed between atoms having different electronegativities. the atom with higher electronegativity will have greater tendency to attract the shared pair of electrons towards itself than the other atom. As a result, the cloud of shared electron pair gets distorted and polar covalent bond is formed.

2. Example – HF – Hydrogen fluoride: 

The electronegativities of hydrogen and fluorine on Pauling’s scale are 2.1 and 4 respectively. It means that fluorine attract the shared pair of electrons approximately twice as much as hydrogen which leads to partial negative charge on fluorine atom and partial positive charge on hydrogen atom. Hence, the H – F bond is said to be a polar covalent bond.

(B) Consists of molecules

(1) Water → covalent bond.

(2) Calcium oxide → ionic bond or electrovalent bond

(i) Covalent bond

(ii) Ionic bond

(iii) Covalent bond

(iv) Covalent bond

(v) Covalent bond

(vi) Ionic bond

(vii) Covalent bond

(viii) Covalent bond

(ix) Covalent bond

(xi) Covalent and co-ordinate bonds

(x) Covalent bond 

a. Water - Covalent bond

b. Calcium oxide - Ionic bond

c. Hydroxyl ion - Polar covalent bond

d. Methane - Covalent bond

e. Ammonium ion - Co-ordinate covalent bond

f. Ammonium chloride - Covalent, coordinate and ionic bonds

As the size of the cation decrease, lattice enthalpy increases. Li⁺ ion is smaller than Na⁺ ion. 

Hence, 

Lattice enthalpy of LiF is more than that of NaF.

Linus Pauling

In a covalent bond the relative ability of each atom to attract the bonded pair of electrons towards itself is called electronegativity.

Salt solution is good conductor of electricity because common salt (NaCl) is an ionic compound and the rest are covalent compounds.

Correct option is  D) Na

Correct option is  B) O^2-

HCl  + H₂O →   H₃O⁺ + Cl^-

HBr + H₂O →  H₃O⁺  + Br^-

When two oppositely charged ions are engaged in a bond, it is known as ionic linkage.

The number of electrons lost from a metal atom is the valence of its element which is equal to its group number. 

Ex : Na and Mg have valence 1 and 2 respectively.

Option : c. MOT

Option : d. H₂O

Due to the electron transfer the following changes occurs –

(i) Both the atoms acquire stable noble gas configuration.

(ii) The atom that loses electrons becomes +vely charged called cation whereas that gains electrons becomes –vely charged called anion.

(iii) Cation and anion are held together by the coulombic forces of attraction to form an ionic bond.

Lone pair of electrons do not take part in bond formation where as bonded pair of electrons take part in bond formation.

Bond order is defined as number of bonds between two atoms in a molecule.

Examine the above table and identify how to write the chemical formula from valency. 

Compare your findings with the following. 

• First write the element with lower electronegativity. 
• Exchange the valency of each element and write as suffix. 
• Divide the suffix with the common factor. 
• If the suffix is 1, it need not be written.

a) The electronic configuration of Be is 1s² 2s². So its valency is 2.

b) The electronic configuration of Nitrogen is 1s² 2s² 2p³. So its valency is 3. 

∴ Formula of the compound is Be₃N₂.

Valency is the combining capacity of the atoms of an element. It can be treated as the number of electrons lost gained or shared by an atom during chemical combination.

There are 2 electrons

Bonding molecular orbital has lower energy and hence greater stability than the corresponding antibonding molecular orbital.

It will form a cation : M³⁺

M₂(SO₄)₃

M(NO₃)₃

M₃(PO₄)₃

M₂(CO₃)₃

M(OH)₃

Atoms combine with other atoms to attain stable octet or noble gas configuration

They have 8 (e^-) electrons in their outermost orbit.