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Chemical Bonding And Molecular Structure

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Chemical Bonding and Molecular Structure PDF Notes, Important Questions and Synopsis

SYNOPSIS

  • The attractive force which holds various constituents (atoms and ions) together in different chemical species is known as chemical bond.
  • Octet Rule-According to the electronic theory of valence, every atom tries to attain the octet configuration (presence of eight electrons) in its valence shell by losing, gaining or sharing of electrons. This is known as the octet rule
  • Lewis Symbols- To understand the concept of valence electrons, Lewis introduced the concept representing valence electrons with dots, which are called Lewis symbols.
  • Electrovalent Bond- The bond formed as a result of the electrostatic attraction between positive and negative ions is termed the electrovalent bond. The electrovalence is thus equal to the number of unit charge(s) on the ion.
  • Covalent Bond- When two same or different atoms share their valence electron to attain the noble gas configuration, it is known as a covalent bond.
  • Formal Charge- The formal charge of an atom in a polyatomic molecule or ion may be the difference between the number of valence electrons of that atom in an isolated or free state and the number of electrons assigned to that atom in the Lewis structure.

    begin mathsize 12px style table attributes columnalign left end attributes row cell Formal text    end text charge text     end text on text    end text an text    end text atom text    end text in text    end text straight a text    end text Lewis text    end text structure text    end text equals end cell row cell open square brackets table attributes columnalign left end attributes row cell Total text    end text number text    end text of text    end text valence end cell row cell electrons text    end text in text    end text the text    end text free text    end text atom end cell end table close square brackets text    end text minus text    end text open square brackets table attributes columnalign left end attributes row cell Total text    end text number text    end text of text     end text non text    end text bonding text    end text end cell row cell left parenthesis lone text     end text pair right parenthesis text    end text electrons end cell end table close square brackets text    end text minus text    end text open square brackets 1 half close square brackets text    end text open square brackets table attributes columnalign left end attributes row cell Total text    end text number text    end text of text    end text bonding end cell row cell left parenthesis shared right parenthesis text    end text electrons end cell end table close square brackets end cell end table end style
  • Hybridisation: Mixing of two atomic orbitals with the same energy level to give a degenerated new type of orbitals Bond Length- Bond length is the equilibrium distance between the nuclei of two bonded atoms in a molecule.
  • Bond Angle- Bond angle is the angle between the orbitals containing bonding electron pairs around the central atom in a molecule/complex ion.
  • Bond Enthalpy- Bond enthalpy is the amount of energy required to break one mole of bonds of a particular type between two atoms in a gaseous state.
  • Bond Order- According to Lewis, bond order is given by the number of bonds between two atoms in a molecule.
  • Valence Bond Theory- Valence bond theory says that electrons in a covalent bond reside in a region which is at the overlap of individual atomic orbitals.
  • Sigma (σ) bond- The covalent bond formed because of overlapping of atomic orbitals along the internuclear axis is called the σ-bond.
  • Pi (π) bond- The covalent bond formed by sidewise overlapping of atomic orbitals is called π-bond.
  • VSEPR Theory- The Lewis concept is unable to explain the shapes of molecules. Sidgwick and Powell provided a useful idea for predicting shapes and geometries of molecules. The theory was based on the repulsions between electron pairs, known as valence shell electron pair repulsion (VSEPR) theory.

 

e Pairs

 

Notation

Name of VSEPR shape

Examples

Reason for the acquired shape

2

AX2

Linear

HgCl2, ZnI2, CS2, CO2

 

3

AX3

Trigonal planar

BF3, GaI3

 

AX2E

Non-linear

SO2, SnCl2

Theoretically, the shape should have been triangular planar, but actually it is bent or v-shaped. The reason being the lone pair–bond pair repulsion is much more as compared to the bond pair–bond pair repulsion. So, the angle is reduced to 119.5° from 120°.

4

AX4

Tetrahedral

CCl4, CH4,

 

AX3E

Trigonal pyramidal

NH3,

Had there been a bp instead of a lp, the shape would have been tetrahedral; but one lone pair is present, and because of the repulsion between lp–bp (which is more than bp–bp repulsion), the angle between the bond pairs is reduced to 107° from 109.5°.

AX2E2

Non-linear

H2O, SeCl2

The shape should have been tetrahedral if there were all bp, but two lp are present, so the shape is distorted tetrahedral or angular. This is because

lp–lp repulsion is more than lp–bp repulsion which is more than bp–bp repulsion. Thus, the angle is reduced to 104.5° from 109.5°.

5

AX5

Trigonal bipyramidal

PCl5, PF5

 

AX4E

Distorted tetrahedral

TeCl4, SF4

The lp is in an equatorial position, and there are two lp–bp repulsions. Hence, it is more stable.

AX3E2

T-shaped

ClF3, BrF3

The lp is at an equatorial position, so there are less lp–bp repulsions as compared to others in which the lp is at the axial position. So, a T-shaped structure is the most stable.

AX2E3

Linear

,

 

6

AX6

Octahedral

SF6,

 

AX5E

Square pyramidal

IF5, BrF5

 

AX4E2

Square planar

begin mathsize 12px style ICl subscript 4 superscript minus end stylebegin mathsize 12px style BrF subscript 4 superscript minus end style

 

  • Hybridisation: Mixing of two atomic orbitals with the same energy level to give a degenerated new type of orbitals.

                 Hybridisation

                        Geometry

                      sp

                          Linear

                      sp2

                          Trigonal Planar

                     sp3

                          Tetrahedral

                      sp3d

                          Trigonal bipyramidal

                      sp3d2

                           Octahedral

                      sp3d3

                          Pentagonal bipyramidal

  • Molecular Orbital Theory- According to Molecular Orbital Theory, individual atoms combines to form molecule. Thus the electrons of an atom are present in various atomic orbitals and are associated with several nuclei.
  • Energy Level Diagram for Molecular Orbitals 
    Contours and energies of bonding and antibonding molecular orbitals formed through combinations of 1s atomic orbitals, 2pz atomic orbitals and 2px atomic orbitals are shown below.





  • Hydrogen Bond- The hydrogen bond is  a  special  type  of  attraction  between  a  hydrogen  atom  bonded  to  a  strongly electronegative atom (such as nitrogen, oxygen or fluorine) and the unshared pair of electrons on another electronegative atom.

  • Vander wall Interactions- Vander wall interactions are weakest of all bondings. These are driven by induced electrical interaction between two or more atoms or molecules.

  • Lattice Enthalpy- The lattice enthalpy of an ionic solid is the energy required to completely separate one mole of a solid ionic compound into gaseous constituent ions.

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