Chemical bonding

• A chemical bond is an interatomic force of attraction that holds two atoms together in a molecule
• According to Lewis, atoms of different elements combine to complete their octets or duplets to attain stable electronic configuration
• Atoms can combine by transferring valence electrons or by sharing valence electrons to attain their octet, known as the octet rule
• Ionic bond is formed by the complete transfer of one or more electrons from one atom to another
• Conditions for the formation of an ionic bond:

  • Atoms forming positive ions (usually metals) should have low ionization energy, low electron affinity, low electronegativity, and high lattice energy
  • Atoms forming negative ions (usually non-metals) should have high electron affinity, high electronegativity, high ionization energy, and high lattice energy
• Properties of ionic compounds:

  • Ionic compounds are soluble in water
  • Ionic compounds usually have high melting points
  • Ionic bond is non-directional, strength depends on distance but not direction
• A covalent bond is formed when atoms combine by mutual sharing of electrons
• Steps for Lewis Representation of Simple Molecules:

  1. Count valence electrons of all atoms

  2. Add or subtract electrons for charges

  3. Identify least electronegative atom and place it at the center

  4. Place other atoms evenly around it

  5. Draw single bonds to the central atom

  6. Complete octet for outer atoms and add remaining electrons to central atom

  7. Complete octet for central atom with remaining electrons
• Limitations of the Octet Rule:

  • Incomplete octet of the central atom
  • Odd-electron molecules
  • Expanded octet
• Bond Parameters:

  • Bond length: distance between nuclei of bonded atoms
  • Bond angle: angle between orbitals containing bonding pair of electrons
  • Bond enthalpy: energy required to break one mole of bonds
  • Bond order: number of bonds between two atoms in a molecule
• Resonance Structures:

  • Ozone O3
  • Carbonate CO3-2
  • Carbon Dioxide CO2
• Types of Covalent Bond:

  1. Sigma(σ) bond: formed by axial overlap of atomic orbitals

  2. Pi(π) bond: formed by sidewise overlapping of atomic orbitals
• Hybridisation:

  • Process of intermixing atomic orbitals to form equal number of new orbitals of same energy
  • Salient features: equivalent energy and shape, more effective in forming stable bonds, directed in space to minimize repulsion
  • Type of hybridisation indicates the geometry of molecules
• Types of Hybridisation:

  1. sp3

  2. sp2

  3. sp
• sp3 hybridisation for methane CH4:

  • During formation, 2s orbital and 2px, 2py, 2pz orbitals undergo sp3 hybridization to form four identical sp3 hybrid orbitals
  • The four sp3 hybrid orbitals of carbon atom are directed towards the four corners of the tetrahedron with an angle of 109.28°
• sp3 hybrid orbitals of carbon atom are directed towards the four corners of the tetrahedron with an angle of 109.28°
• Each sp3 hybrid orbital of carbon atom overlaps with 1s orbital of hydrogen atom to form four sigma bonds in methane molecule
• In ethene, two carbon atoms undergo sp2 hybridization
• One sp2 hybrid orbital of one carbon atom overlaps with sp2 hybrid orbital of the second carbon atom to form a sigma bond
• The other two sp2 hybrid orbitals of each carbon atom overlap with 1s orbital of the hydrogen atom to form four sigma bonds
• The unaffected 2pz orbitals overlap sideways to form one pi bond in ethene
• In acetylene molecule, two carbons undergo sp hybridization separately
• One sp hybrid orbital of one carbon atom overlaps with another sp hybrid orbital of the other carbon atom to form a sigma bond
• The other sp hybrid orbital overlaps with 1s orbital of hydrogen atom to form two sigma bonds
• The unaffected 2py and 2pz orbitals overlap sideways to form two pi bonds in acetylene
• In beryllium chloride (BeCl2) formation, 2s orbital and 2px orbitals of beryllium atom undergo sp hybridization to form two identical sp hybrid orbitals
• These two sp hybrid orbitals are oriented in opposite directions with an angle of 180°
• Each sp hybrid orbital of one beryllium atom overlaps with 2pz hybrid orbital of the chlorine atom to form a sigma bond
• In the formation of PCl5, phosphorous undergoes sp3d hybridization
• The shape of PCl5 is trigonal pyramidal
• In the formation of SF6, sulfur undergoes sp3d2 hybridization
• The shape of SF6 is octahedral
• VSEPR theory postulates that the geometry of a molecule depends on valence shell electron pairs around the central atom
• The geometry of molecules depends on the repulsion between electron pairs, with LP-LP repulsion being the strongest
• Examples of molecular geometries include linear, trigonal planar, tetrahedral, trigonal bipyramidal, and octahedral
• Dipole moment is defined as the product of the magnitude of the charge and the distance between the centres of positive and negative charge
• The dipole moment of water (H2O) is 1.85 Debye or 6.17 x 10^-30 C m
• The dipole moment of ammonia (NH3) is 4.90 x 10^-30 C m
• The dipole moment of nitrogen trifluoride (NF3) is 0.80 x 10^-30 C m
• NF3 molecule has a dipole moment of 0.80 x 10 - 30 C m