www.free-education.in is a platform where you can get pdf notes from 6th to 12th class notes, General Knowledge post, Engineering post, Career Guidelines, English Speaking Trick, How to crack an interview and lots more. ( Chapter 4 Chemical Bonding and Molecular Structure Notes and NCERT Solution )
|Section Name||Topic Name|
|4||Chemical Bonding and Molecular Structure|
|4.1||Kössel-Lewis Approach to Chemical Bonding|
|4.2||Ionic or Electrovalent Bond|
|4.4||The Valence Shell Election Pair Repulsion (VSEPR) Theory|
|4.5||Valence Bond Theory|
|4.7||Molecular Orbital Theory|
|4.8||Bonding in Some Homonuclear Diatomic Molecules|
We have studied about so many elements till now .Out of 114 elements, most of them occur in combined state. Only noble gases occur in atomic state otherwise, most of them exist in molecular or combined form.
The combined state is formed by combination of elements that is a chemical bond. Like families, have connections in the same way the attraction of chemical species is called chemical bond.
The reason behind the formation of chemical bond is to obey octet rule .According to this rule, “every element needs to attain stability. For this, the elements either loose or gain electrons to attain 8 electrons in its outermost shell” .But certain violation were seen like in case of hydrogen, the atom has tendency to attain only 2 electrons in valence shell .So, in case of it the rule is violated and is called as Duplet rule.
Other violations are: In certain molecules, the deficient octet is seen. For example in H2 ,whereas in others the central atom has expanded octet that is more than 8 electrons like in SF6.
When two atoms come closer, their outer electrons repel each other and finally they stop at a distance where the released energy is more .As less energy corresponds to maximum stability than at that distance the chemical bond is formed.
Types of chemical bond
- Electrovalent or ionic bond
- Covalent bond
- Coordinate bond
Ionic bond and factors affecting it
- It is formed by transfer of electrons.
- It is formed between metal and no metal.
- In it metal always loses electrons and non-metal always accepts electron.
- Sodium chloride
Factors affecting the formation of ionic bond
The factors are:
- Low ionization energy
- High electron affinity
- High lattice enthalpy
- Low ionization energy: The metals with low ionization energy favor the formation of ionic bond. As lower is the ionization energy more readily it will lose electrons.
- High electron gain enthalpy: The non-metal participating should have high electron gain enthalpy because more it will have attraction, for upcoming electron more readily the bond will be formed.
- Lattice enthalpy: It is the amount of energy needed to break one mole of bonds into its constituents, or the energy released when constituents combine to form on 1 mole of a compound. More is the lattice energy, more stable is the bond formed. All the compounds in which ionic bond is present are called as ionic compounds.
General properties of ionic compounds:
- Physical state: They form definite pattern that is crystal lattice. Crystal lattice is 3D arrangement of cation and an anion.
For example, in NaCl crystal due to crystal formation they all are solids due to strong bonding between constituents.
- Melting and boiling point: They have high melting and boiling points because of strong attraction between constituents.
- Solubility: We know like dissolves like. So, polar compounds are soluble in polar solvents. Now, ionic compounds have a charge that is they are polar. Therefore, they will dissolve in polar solvents like water. So, all ionic compounds are soluble in polar solvents and insoluble in organic solvents.
- Electrical conductivity: It is due to free movement of free ions when ionic compound is dissolved in water. When dissolved they break into ions and conduct electricity.
- Non directional in nature: When we are talking of directions in 3D structure, we are talking about 3 coordinate .So, in NaCl or any other ionic compounds the ion can take place in any direction .There direction is not fixed. Therefore, they are non-directional in nature.
They are formed by mutual sharing of electrons between combining atoms
Note: These all compounds are neutral that is they have no charge on them
They are the measurable properties of a bond and these are associated only with covalent bond.
- Bond length: It is the average distance between the centre of two bonded atoms .
Factors on which bond length depends:
- Size of the atoms: Bigger the size of the atom more is the bond length.
- Multiplicity of bond: If there is a single bond, bond length is longer as compared to bond length in double or triple bond.
- That is the reason bond length of alkanes is more than alkenes which is further more than alkynes.
- Bond angle: It is an average distance between the orbital of the atoms surrounding the central atom.
- Bond order: It is the number of bonds present between two atoms.
For example: In H2 (bond order is 1 )
In oxygen molecule (bond order is 2)
In nitrogen molecule bond order is 3)
- In Iso-electronic species: The bond order is same.
Example: In Fluorine and O22- the electrons are 18 and bond order is same that is 1.
- Resonance averages the bond characteristics of a molecule as a whole.
- Resonance stabilizes the molecule because energy of the resonance hybrid is less than the energy of any canonical forms.
- It has no real existence.
- Resonance has the same bond length.
- A resonance hybrid has the lowest energy.
- The greater the resonance and resonance energy the more stable is the structure.
- Resonance is a theoretical concept with no experimental verification.
Polarity & Dipole moment
In homo-atomic molecules having same type of atoms are present .In them sharing of electrons occur the shared pair remain in the middle that is equal attraction by atoms towards shared pair.
- The hetro-atomic molecules (having different types of atoms in them) when sharing of electrons occur the shared pair do not lie in middle, it gets displaced towards more electronegative atom.
- So, it will acquire partial negative charge and other atom will acquire partial positive charge. Due to this the poles are developed and the bond formed is polar covalent bond.
Let us calculate the dipole moment of different molecules with different atomicity. Dipole moment tells us about the structure of molecule.
- If dipole moment is zero, than the molecule is non polar.
- If dipole moment is not zero, than the polar molecule.
Dipole moment and molecular structure
Let us consider:
- Diatomic molecule: Its structure is generally linear.
Example:- In HCl, dipole moment is : 1.07 D
In HBr, dipole moment is : 1.78 D
In Hl, dipole moment is : 0.79 D
- More is the magnitude of charge: More is the covalent character.
For Example:- NaCl< CaCl2 < AlCl3
(Maximum covalent character in aluminium chloride)
- Electronic configuration of cation: Cation with 18 electrons in its outermost shell can cause more distortion.
For Example:- Na+ = 1s2, 2s2, 2p6
Cu+ = 1s2,2s2,2p6,3s2,3p6,4s1,3d10
Out of NaCl and CuCl, the latter has more covalent character.
Geometry of molecule
- It is the relative arrangement of bonded atoms in a molecule.
- The way in which the bonded atoms direct themselves around the central atom is explained on the basis of this theory.
- This theory was given by Sid wick and Favell.
- In this only the valence electrons participate in bond formation.
- The bonded atoms around central metal atom will have repulsions between them
- Therefore, they will arrange themselves in such way that the repulsion is minimum.
In BF3 the arrangement is like it is shown below:
- Number of hybrid orbitals is equal to number of atomic orbitals that combine.
- The hybrid orbitals are always equal in shape and energy.
- The hybrid orbitals are more effective in forming bonds as compared to pure atomic orbitals.
- The hybrid orbitals are directed towards specific directions in space.
- The type of hybridization gives us the shape of molecule
Now there is a question that can all participate in hybridization
Conditions of hybridization
- The only valence orbitals participate.
- The atomic orbitals that participate should have almost same energy.
- Promotion is not always necessary.
- The unpaired as well as fully filled orbitals can also participate.
Types of hybridization
- sp hybridization: In this one s and one p orbital combines as shown below.
Valence bond approach of covalent bond
VSEPR theory does not tell us about bond parameters like directional nature, bond angle, repulsions etc.
To explain we have new theory:
- Valence bond approach
- Molecular orbital theory
- Valence bond theory
- According to this, the atom retains their identity even after bonding.
- Bond is formed due to interaction of valence electrons only.
- Only the valence electrons lose their identity whereas inner electrons do not participate.
- Stability of bond depends upon amount of energy released
- The molecule has minimum energy at a specific distance called inter-nuclear distance and at that the bond formation takes place.
Overlapping: sigma and pi bond
Overlapping can be defined as partial merging of bonded orbitals .More is the overlapping, stronger is the bond formed.
Types of overlapping
- Head to head
Accordingly, the bond formed is sigma or pi bond.
- In this head to head overlapping occurs.
- More is the overlapping region more stable is the bond.
- The bond is stronger.
- This bond can exist independently.
- It is formed by side wise overlapping of orbitals.
- It is not too strong as in this overlapping region is less.
- It is weaker bond as compared to sigma bond.
- It can’t exist independently, it exist along with sigma bond.
Let’s make Sigma bond :
- Molecular orbital theory
It was developed by F.Hund and R.S Mullikan in 1932.
The features of this theory are:
- The electrons in a molecule are present in various molecular orbital as the electrons of atom are present in different various shells.
- The atomic orbitals of comparable energies and proper symmetry combine to form molecular orbitals.
- In molecular orbital electrons are in influence of two or more nuclei.
- The number of molecular orbitals formed is equal to number of atomic orbitals that combine.
- The two orbitals formed due to combination are: Bonding and Anti -bonding.
- The Bonding molecular orbital has lower energy and greater stability than Anti -bonding.
- The electron probability distribution around group of nuclei is given by molecular orbital.
- The molecular orbitals are filled in accordance to Aufbau principle, Pauli’s principle and Hund’s rule.
The linear combination of atomic orbitals to form molecular orbital takes place only if:
- The combining orbitals must have same energy.
- The combining orbitals must have same symmetry.
- The combining orbitals must overlap to maximum extent.
The order to fill molecular orbital is:
- We can find the bond order : That is
Bond Order 1 (single bond)
Bond Order 2 (double bond)
Bond Order 3 (triple bond)
( c) Tell us about the type of bond :
- Magnetic character :
If unpaired electrons – paramagnetic
If no unpaired electrons – dimagnetic
Hydrogen bond is formed when hydrogen is attached to some electronegative element like O, N and F. It forma a special bond with them called as hydrogen bond.
Types of Hydrogen bond
- Intermolecular H bond.
- Intra-molecular H bond.
Intermolecular Hydrogen bond: It is a bond formed between two different molecules.
Intra-molecular Hydrogen bond: A bond that is formed within the molecule.
Consequences of Hydrogen bonding:
- They exist as associated molecules.
- Boiling points of compound having H-bond are higher as they have extra bonds in them.
- They generally exist as liquids.
Hydrogen bonding in ice and water
The structure of Ice is given below :
- In it each oxygen atom is linked to 4 Hydrogen.
- Two Hydrogen are linked by covalent bond and 2 Hydrogen by Hydrogen bonds.
- It forms network and space between is hollow.
- That is why; the ice is lighter due to hollow space.
- Therefore, ice floats on water.
- As we know, ice has high density so it should sink
- But, in actual practice it does not sink.
- Due to presence of hydrogen bonding in it, this makes the structure of ice hollow.
- Therefore, density decreases.
Water has maximum density at 4 degree Celsius:
This is seen because at zero degrees Celsius the water is ice. Now, if we start increasing its temperature gradually, we see that the cage of ice starts breaking. Due to it, hollow space starts getting filled. H-bonds break therefore, density increases.
At 4 degree Celsius, kinetic energy of molecules starts increasing and they start moving away due to which density decreases.
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