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CHEMICAL Bonding

 

KEY CONCEPTS

 

INTRODUCTION

A molecule will be formed if it is more stable and has a lower energy than the individual atom. Normally only electrons in the outermost shell of an atom are involved in forming bonds, and in this process each atom attains a stable electronic configuration of an inert gas.

Types of Bonds :

Atoms involved Bonding

(I) Metal Nonmetal Ionic

(II) Nonmetal Nonmetal Covalent

(III) Metal Metal Metallic

(IV) Electron deficient molecule or ion (Lewis acid) and Coordinate

electron rich molecule or ion (Lewis base)

(V) H and electronegative elements (N, O, F some times with Cl) Hydrogen

Ionic, Covalent and coordinate bonding

Ionic bonding

An ionic bond is formed when a metal atom transfers one or more electrons to a non-metal atom.

Chemical Bonding and Molecular Structure, Chapter Notes, Class 11, Chemistry

The oppositely charged ions get attracted through electrostatic force of attraction.

Properties of ionic compounds :

Chemical Bonding and Molecular Structure, Chapter Notes, Class 11, Chemistry In the solid state, each cation surrounds itself with anions and each anion with cations. These very large number of ions are arranged in an ordinary network called ionic crystals.

Chemical Bonding and Molecular Structure, Chapter Notes, Class 11, Chemistry They are good conductors of electricity in fused state or aqueous solution.

Chemical Bonding and Molecular Structure, Chapter Notes, Class 11, Chemistry They are generally soluble in polar solvents and insoluble in non-polar solvents.

Chemical Bonding and Molecular Structure, Chapter Notes, Class 11, Chemistry Have high melting point and boiling point than covalent compounds.

Chemical Bonding and Molecular Structure, Chapter Notes, Class 11, Chemistry Have strong force of attraction between cation and anion (Coulombic force)

F = Chemical Bonding and Molecular Structure, Chapter Notes, Class 11, Chemistry

Where Z1 and Z2 are charge on the ions, D is dielectric constant of the medium and r the inner-ionic distance (i.e., bond length).

Born-Haber cycle It is an indirect method to calculate the lattice energy of an ionic compound. For example, the lattice energy of sodium chloride can be calculated as follows.

Chemical Bonding and Molecular Structure, Chapter Notes, Class 11, Chemistry

Now, according to Hess's law,

Chemical Bonding and Molecular Structure, Chapter Notes, Class 11, Chemistry = S IE1 D/2 _ EA1 _ U

Where S is the enthalpy of sublimation of metal (Na), IE1 is the first ionisation energy of sodium, D is the bond dissociation energy of Cl1 molecule, EA1 is the first electron affinity of Cl, U is the lattice energy of NaCl(s) and DHformation is the enthalpy of formation of NaCl.

Covalent Bonding :

Whenever Chemical bond is formed by sharing of electrons then it is named as covalent bond.

Coordinate Bonding or Dative Bond :

Whenever covalent bond is formed by unequal sharing of a pair of electrons between a Lewis base and Lewis acid is called coordinate bond.

(I) It is represented as (¾¾®) and considered as s bond.

(II) Atom/ion/molecule donating electron pair is called donor or Lewis base.

(III) Atom/ion/molecule accepting electron pair is called acceptor or Lewis acid.

Formation of NH4 :

NH3 has three (N _ H) bonds and one lone pair. In NH4 formation. This lone pair is donated to H ion.

: NH3 H ¾® NH4

Chemical Bonding and Molecular Structure, Chapter Notes, Class 11, Chemistry

Note : In a multiple bond (Chemical Bonding and Molecular Structure, Chapter Notes, Class 11, Chemistry) sign is treated as a pi-covalent bond (Bock bond).

Valence Bond (V.B) theory :

According to this theory, a covalent bond is formed by the overlapping of atomic orbitals. Important points of this theory are summarised below.

(I) Orbitals undergoing overlapping should be half-filled.

(II) Half-filled orbitals should contain the electron with opposite spin.

(III) Strength of a covalent bond depends upon the extent of overlapping, for example, axial or lateral overlapping.

(IV) If the atomic orbitals overlap axially, then the bond formed is called a sigma (s) bond.

(V) A sigma bond is always stronger than a pi-bond.

(VI) Covalent bonds formed by the overlap of s-s and s-p orbitals are always sigma.

(VII) By the overlap of p-p orbitals, one sigma and two pi bonds are formed.

(VIII) Increasing strength of s covalent bonds is in the order s _ s < s - p < p — p (when internuclear distance is constant)

(IX) A single covalent bond is always a sigma bond. In a double covalent bond, one is sigma and the other is pi-bond. In a triple covalent bond, one is sigma and two are pi-bonds.

Hybridisation :

It is defined as the concept of intermixing of orbitals of same energy or of slightly different energy to produce entirely new orbitals of equivalent energy, identical shapes and which are symmetrically disposed is plane.

Important features of hybridisation are given below.

(I) Only the orbitals generated are equal in number to that of pure atomic orbitals which are intermixed.

(II) The hybrid orbitals generated are equal in number to that of pure atomic orbitals which are intermixed.

(III) A hybrid orbital, like an atomic orbital, can have two electrons of opposite spins.

(IV) Hybrid orbitals usually form sigma bonds. If there are pi-bonds, equal number of atomic orbitals must be left unhybridised for pi-bonding.

Valence shell electron pair repulsion (VSEPR) Theory (Gillespie theory) :

(I) The shape of a molecule is determined by repulsion between the electron pairs (lp and bp) present in the valence shell of the central atom.

(II) The order of repulsion is (lp-lp) > (lp-bp) > (bp-bp)

Determination of the Type of Hybridisation :

The number of electron pairs = Bond Pairs lone pairs

(s Bond)

Table Shapes of molecules on the basis of VSEPR theory and hybridisation

Chemical Bonding and Molecular Structure, Chapter Notes, Class 11, Chemistry

Molecular Orbitals (M.O.) Theory :

(I) There is Linear combination of Atomic Orbitals (LCAO) to form molecular orbitals (M.O.)

(II) The number of molecular orbitals produced is equal to the number of atomic orbitals that are combined.

(III) Two types of molecular orbitals are formed.

(a) Bonding molecular orbitals at a lower energy than the combining atomic orbitals which favour bond formation.

(b) Anti - bonding molecular orbitals at a higher energy than the combining atomic orbitals which opposes the bond formation.

Following molecular orbitals are formed when atomic orbitals overlap.

Atomic orbitals Bonding M. O. Anti-bonding M.O.

1s 1s s 1s s* 1s

2s 2s s 2s s* 2s

2px 2px s 2px s* 2px

2py 2py p 2py p* 2py

2pz 2pz p 2pz p* 2pz

Electrons are filled in the increasing order of energy of the molecular orbitals which is in order of Aufbau rule

(a) When total electrons in the molecule or ions are more than 14(simple order of energy level of M. O.).

Chemical Bonding and Molecular Structure, Chapter Notes, Class 11, Chemistry

(b) When total number of electrons in the molecule are ions are £14 (modified order of energy level of M.O.).

Chemical Bonding and Molecular Structure, Chapter Notes, Class 11, Chemistry

(I) The electrons are filled in molecular orbitals in the same way as in atomic orbitals, that is, as per the Hund's Rule, Pauli exclusion principle and Aufbau rule.

(II) Number of covalent bonds between two atoms is called Bond order and is given by

Bond order (B.O.) = Chemical Bonding and Molecular Structure, Chapter Notes, Class 11, Chemistry

Where, NB = number of electrons in bonding molecular orbital and NA = number of electrons in anti bonding molecular orbitals.

(III) For a stable molecule/ion, NB > NA

(IV) A given molecule or ion is paramagnetic if it has unpaired electrons in the molecular orbital. Otherwise it is diamagnetic

(V) Greater the bond order, shorter the bond length and greater the bond energy. Thus for O2, O2 , O2,O22_

Chemical Bonding and Molecular Structure, Chapter Notes, Class 11, Chemistry

Fajan's Rule :

This rule is used to decide relative ionic & covalent character in a molecule. A molecule is predominantly covalent if

(I) Smaller the size of cation.

(II) larger the size of anion.

(III) greater the charge on cation and anion.

(IV) ion does not have inert gas configuration but it possesses pseudo inert gas configuration (18 electrons in the ultimate shell).

(I) Chemical Bonding and Molecular Structure, Chapter Notes, Class 11, Chemistry

(II) Chemical Bonding and Molecular Structure, Chapter Notes, Class 11, Chemistry

(III) Chemical Bonding and Molecular Structure, Chapter Notes, Class 11, Chemistry

(IV) CuCl and NaCl

[Cu ] = [Ar]3d10 ; [Ne ] = [Ne]

Cations with 18-electron shells (pseudo inert gas configuration) has grater polarising power than 8-electron shell (inert gas configuration) ion with the same charge and size. Thus, CuCl is more covalent than NaCl.

Hydrogen Bonding :

(I) The concept of H-bonding was introduced by Latimer and Rodebush.

(II) H-bonding is said to be formed when slightly acidic hydrogen attached to a strongly electronegative atom such as F, O and N is held with weak electrostatic forces by the non- bonded pair of electron of another atom. That is , it is a dipole-dipole interaction.

(III) Of all the electronegative donor atoms, only F, O. and N enter into stable H-bond formation.

(IV) The weak electrostatic interaction leading to the H-bond formation is shown by dashed(.....) lines.

(V) Greater the electronegativity difference, stronger is the H-bond. For example,

Chemical Bonding and Molecular Structure, Chapter Notes, Class 11, Chemistry

Intramolecular H-bonding ;

This type of H-bonding occurs when polar H and electronegative atom are present in the same molecule.

(a) o-hydroxy benzaldehyde (b) o-nitrophenol

Chemical Bonding and Molecular Structure, Chapter Notes, Class 11, Chemistry Chemical Bonding and Molecular Structure, Chapter Notes, Class 11, Chemistry

 

Intermolecular H-bonding

This type of H-bonding takes place between hydrogen and electronegative element (like F,O,N) present between molecules of the same substance (H2O and H2O) or different substances (H2O and NH3).

 

 

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FAQs on Chemical Bonding and Molecular Structure, Chapter Notes, Class 11, Chemistry

1. What is a chemical bond?
Ans. A chemical bond is a force that holds atoms together in chemical compounds. It is a result of the interaction between the valence electrons of atoms.
2. What are the types of chemical bonds?
Ans. There are three types of chemical bonds - ionic bonds, covalent bonds, and metallic bonds. Ionic bonds occur between a metal and a non-metal, covalent bonds occur between two non-metals, and metallic bonds occur between two metals.
3. What is the difference between ionic and covalent bonds?
Ans. The main difference between ionic and covalent bonds is the way in which the atoms are bonded. In an ionic bond, one or more electrons are transferred from one atom to another, resulting in the formation of ions. In a covalent bond, atoms share electrons to form a molecule.
4. What is the VSEPR theory?
Ans. The VSEPR theory (Valence Shell Electron Pair Repulsion theory) is a model used to predict the geometry of molecules based on the number of valence electrons and the repulsion between electron pairs. It states that the electron pairs around the central atom will arrange themselves in a way that minimizes repulsion.
5. What is hybridization?
Ans. Hybridization is a concept used to describe the mixing of atomic orbitals to form new hybrid orbitals in a molecule. This theory is used to explain the structure of molecules and the properties of chemical bonds in terms of the overlap of atomic orbitals.
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