H₂ molecule :
Electronic configuration of H atom 1s¹
Electronic configuration of H₂ molecule \(\sigma^{2}_{1s}\)
Bond order = \({N_b-N_a\over2}={2-0\over2}=1\)
Molecule has no unpaired electrons hence it is diamagnetic

Helium molecule (i.e) He₂ :
The electronic econfiguration of He atom is 1 s²
.'. Electronic configuration of He₂ molecule is \((\sigma_{1s})^2(\sigma^*_{1s})^2\)
Bond order = \({N_b-N_a\over2}={2-2\over2}\)
= 0
He, cannot exist. Similarly all the inert gases, X₂ cannot orist as their bond order = 0.

Molecules having polar bonds will not necessarily have a dipole moment. For example, the linear form of carbon dioxide has zero dipole moment, even though it has two polar bonds. In CO₂, tlie dipole moments of two polar bonds (CO) are equal in magnitude but have opposite direction. Hence, the net dipole moment of the CO₂ is,
\(\mu=\mu_{1}+\mu_{2}=\mu_{1}+(-\mu_{1})=0.\)


In this case \(\mu=\overrightarrow { { \mu }_{ 1 } } +\overrightarrow { { \mu }_{ 2 } } \quad \)
\(=\overrightarrow { { \mu }_{ 1 } } +\overrightarrow { ({ \mu }_{ 1 }) } =0\)

Dipole moment :
The polarity of a covalent bond can be measured in terms of dipole moment which is defined as \(\mu=q \times 2d\)
Where \(\mu\) is the dipole moment, q is the charge and 2d is the distance between the two charges.
Where p is the dipole moment, q is the charge and 2d is the distance between the two charges. The dipole moment is a vector and the direction of the dipole moment vector points from the negative charge to positive charge.

The unit for dipole moment is columb meter (C m). It is usually expressed in Debye unit (D). The conversion factor is 1 Debye = 3.336 x 10^-30 C m.

Bonding in some heteronuclear di-atomic molecules:
Molecular orbital diagram of Carbon monoxide molecule (CO)
Electronic configuration of C atom 1s²2s²2p²
Electronic configuration of O atom 1s²2s²2p⁴
Electronic conguration of CO molecule
\(\sigma^{2}_{1s},\sigma^{*2}_{1s},\sigma^{2}_{2s},\sigma^{*2}_{2s}\)\(\pi^{2}_{2py},\pi^{2}_{2pz},\sigma^2_{2px}\)
Bond order = \({N_b-N_a\over2}={10-4\over2}=3\)
Molecule has no unpaired electrons hence it is diamagnetic

Molecular orbital diagram of oxyge molecule (O₂) :
Electronic configuration of O atom 1s²2s²2p⁴
Electronic configuration of O₂ molecule
\(\sigma^{2}_{1s},\sigma^{*2}_{1s},\sigma^{2}_{2s},\sigma^{*2}_{2s},\sigma^{2}_{2px},\)\(\pi^{2}_{2py},\pi^{2}_{2pz},\pi^{*1}_{2py},\pi^{*1}_{2pz}\)
Bond order=\({N_b-N_a\over2}={10-6\over2}=2\)
Molecule has two unpaired electrons hence it is paramagnetic.

According to experimental observations, the c-o bonds in \({ CO }_{ 3 }^{ 2- }\)equivalent.
Resonance is mainly responsible for the equal bond lengths of C-O bonds in \({ CO }_{ 3 }^{ 2- }\)

The resonating structure of ozone are:

The central o-atom is considered to have two bond pairs and one lone pair of electrons (ie) it is of AB₂E type.
Hence it is a bent molecule

Hybridisation is the process of mixing of atomic orbitals of the same atom with comparable energy to form equal number of new equivalent orbitals with same energy. The resultant orbitals are called hybridised orbitals and they posses maximum symmetry and definite orientation in space so as to minimize the force of repulsion between their electrons

Lewis Structure of Nitrogen atom
Similarly, Lewis dot structure of carbon, oxygen can be drawn as shown below.

Lewis Structures of C & O atoms
Only exception to this is helium which has only two electrons in its valence shell which is represented as a pair of dots (duet).
\(\overset{..}He\)
Lewis Structures of He atoms

Examples : SF₆, XeF₄
Shapes :
SF₆ - Octahedral
XeF₄ - Square planar

1. AlF₃ is an ionic compound and SiF₄ is a covalent compound.
2. Usually covalent compounds exhibit lower melting and boiling points.
3.\(\therefore\)AlF3 bring ionic in nature has strong coulombic force of attraction between the oppositely charged ions, Thereby posses higher melting point.

The bond energy is defined as the minimum amount of energy required to break one mole of a particular bond in molecules in their gaseous state.

Partial covalent character in ionic:
Like the partial ionic character in covalent compounds, ionic compounds show partial , covalent character. For example, the ionic compound, Iithium chloride shows covalent character and is soluble in organic solvents such as ethanol.
The partial covalent character in ionic compounds can be explained on the basis of a phengmenon called polarisation. We know that in an ionic compound, there is an electrostatic attractive force between the cation and anion. The positively charged cation attracts the valence electrons of anion while repelling the nucleus. This causes a distortion in the electron cloud of the anion and its electron density drifts towards the cation, which results in some sharing of the valence elections between thesi ions. Thus, a partial covalent character is developed between them. This phenomenon is called polarisation.

Molecules having polar bonds will not necessarily have a dipole moment. For example, the linear form of carbon dioxide haszero dipole moment, even though it has two polar bonds. In CO₂, the dipole moments of two polar bonds (CO) are equal in magnitude but have opposite direction. Hence, the net dipole moment of the CO₂ is,
\(\mu=\mu_{1}+\mu_{2}=\mu_{1}+(-\mu_{1})=0.\)

In this case \(\mu=\overrightarrow { { \mu }_{ 1 } } +\overrightarrow { { \mu }_{ 2 } } \quad \)
\(=\overrightarrow { { \mu }_{ 1 } } +\overrightarrow { ({ \mu }_{ 1 }) } =0\)
Incase of water net dipole moment is the vector sum of \(\mu_{1}+\mu_{2}\) as shown.

Dipole moment in water is found to be 1.85 D

Bonding in ethylene:
1. The bonding in ethylene can be explained using hybridisation concept. The molecular formula of ethylene is C₂H₄. The valency of carbon is 4. The electronic configuration of valence shell of carbon in ground state is [He]2s² 2\({ p }_{ x }^{ 1 }\) 2\({ p }_{ y }^{ 1 }\)2\({ p }_{ z }^{ 0 }\). To satisfy the valency of carbon promote an electron from 2s orbital to 2pz orbital in the excited state.

2. In ethylene both the carbon atoms undergoes Sp² hybridisation involving 2s, 2px and 2p orbitals, resulting in three equivalent Sp² hybridised orbitals lying in the xy plane at an angle of 120⁰ to each other. The unhybridised 2pz orbital lies perpendicular to the xy plane.
Formation of sigma bond :
One of the SP² hybridised orbitals of each carbon lying on the molecular axis (x-axis) linearly overlaps with each other resulting in the formation a C-C sigma bond. Other two sp² hybridised orbitals of both carbons linearly overlap with the four 1s orbitals of four hydrogen atoms leading to the formation of two C-H sigma bonds on each carbon.

Formation of pi bond:
The unhybridised 2pz orbital of both carbon atoms can overlap only sideways as they are not in the molecular axis. This lateral overlap results in the formation a pi bond between the two carbon atoms as shown in the figure.

Bonding in acetylene :
1. Similar to ethylene, the bonding in acetylene can also .be explained using hybridisation concept. The molecular formula of acetylene is C₂H₂. The electronic configuration of valence shell of carbon in ground state is [He ]2s² 2\({ p }_{ x }^{ 1 }\) 2\({ p }_{ y }^{ 1 }\)2\({ p }_{ z }^{ 0 }\). To satisfy the valency of carbon promote an electron from 2s orbital to 2pz orbital in the excited state.
2. In acetylene molecule, both the carbon atoms are in sp hybridised state. The 2s and 2px orbitals, resulting in two equivalent sp hybridised orbitals lying in a straight line along the molecular axis (x-axis). The unhybridised 2py and 2pz orbitals lie perpendicular to the molecular axis.
Formation of sigina bond:
1. One of the two sp hybridised orbitals of each carbon linearly overlaps with each other resulting in the formation a C-C sigma bond. The other sp hybridised orbital of both carbons linearly overlap with the two Is orbitals of two hydrogen atoms leading to the formation of one C-H sigma bonds on each carbon. 
Formation of pi bond:
1. The unhybridised 2p_y and. 2p_z orbitals of each carbon overlap sideways. This latera overlap results in the formation of two pi bonds (p_y-p_y and p_z -p_z) between the two carbon atoms as shown in the figure.