chemical bonding

• Atoms or molecules or ions are the basic chemical entities of matter
• Inert gases (He, Ne, Ar, Kr, Xe and Rn) are elements where atoms are capable of independent existence
• Atoms of elements like Na, Mg, O, H, etc, are not capable of independent existence and always exist in a chemically combined state
• During chemical combinations, electronic transactions occur between atoms
• Chemical bond is the attractive force that holds various constituents together in different chemical species
• Atoms of same or different elements combine in different ways depending on various factors
• Kössel – Lewis approach:

  • Valency is the combining capacity of an atom of an element
  • Lewis imagined the atom with a positively charged "Kernel" and an outer shell with a maximum accommodation capacity of 8 electrons (octet)
  • Atoms achieve a stable octet when linked by chemical bonds
  • Chemical bonds are formed by transfer of electrons or sharing of electrons
  • Noble gases do not take part in chemical combinations as they have 8 electrons in their outermost shell
• Lewis symbols represent valence electrons in an atom

  • Only outermost shell electrons are involved in chemical combinations
  • The number of dots in Lewis symbol represents the number of valence electrons
  • Group valence = number of dots in Lewis symbols or 8 - number of dots in Lewis symbols
• Octet rule:

  • Atoms can combine by transfer or sharing of valence electrons to get an octet in their valence shell
• Ionic bond (Electrovalent bond):

  • Formed by transfer of electrons between atoms to attain octet
  • One atom loses electrons to form a positive ion (cation) and the other gains electrons to form a negative ion (anion)
  • Electrovalence is the number of unit charge(s) on the ion or the number of electrons lost or gained by an atom during the formation of an ionic compound
• Characteristics of ionic compounds:

  • Made of ions in solid, liquid, and aqueous state
  • Crystalline solids with high melting and boiling points
  • Insoluble in non-polar solvents and soluble in polar solvents
  • Bad conductors of electricity in solid state, good conductors in molten state and aqueous medium
  • Non-directional in nature and do not exhibit isomerism
• Factors favoring ionic bond formation:

  • Low ionisation enthalpy of electropositive elements
  • High negative electron gain enthalpy of electronegative elements
  • Large difference in electronegativity of metal and non-metal atoms
  • Larger radius of cation and smaller radius of anion favor close packing arrangement in lattice formation
  • Enthalpy of formation of ionic compound should be highly negative
  • High lattice enthalpy favors ionic bond formation
• Lattice Enthalpy:

  • Energy required for the complete dissociation of 1 mole of an ionic solid to gaseous constituent ions to infinite distance
  • Factors associated with crystal geometry have to be included in lattice enthalpy calculation
• Lattice enthalpy is defined as the energy required for the complete dissociation of 1 mole of an ionic solid to gaseous constituent ions to infinite distance
• Example: Lattice enthalpy of NaCl is 788 kJ/mol
• The energy liberated when 1 mole of an ionic crystalline solid is formed from its constituent ions in gaseous state from infinite distance
• In the case of NaCl, the sum of ionisation enthalpy of sodium and electron gain enthalpy of chlorine atom is positive, but the enthalpy of lattice formation is negative, making NaCl solid stable
• Enthalpy of lattice formation provides a qualitative measure of the stability of an ionic compound
• Lattice enthalpy plays a key role in the formation of ionic compounds
• Covalent bond is formed by mutual sharing of electrons between atoms
• A force that binds atoms of the same or different elements by mutual sharing of electrons is called a covalent bond
• Shared electrons become a common property of both atoms and constitute a bond between them
• Formation of Cl2 molecule involves each chlorine atom contributing one electron to share two electrons (shared pair)
• Lewis dot structures provide a picture of bonding in simple molecules and polyatomic ions in terms of shared electron pairs and the octet rule
• Steps for writing Lewis dot structures:

  • Calculate the total number of electrons (T)
  • Calculate the total number of valence electrons (V) of all combining atoms
  • Calculate the number of shared electrons (S) using S = T - V
  • Calculate the number of unshared electrons (U) using U = V - S
  • Write the skeleton structure placing the least electronegative atom in the center
  • Account for shared pairs of electrons for single bonds and remaining electron pairs for multiple bonding or lone pairs
• Formal charge on an atom is the difference between the number of valence electrons in an isolated atom and the number of electrons assigned to that atom in a Lewis structure
• Formal charges help in selecting the lowest energy structure from possible Lewis structures for a given species
• Limitations of octet rule:

  • Formation of compounds with electron deficient atoms
  • Expanded octet or super octet molecules like PCl5, SF6, IF7, etc.
• Examples of compounds where the octet rule does not apply: PF5, SF6, H2SO4, and coordination compounds
• Sulphur forms compounds where the octet rule is obeyed
• In molecules with an odd number of electrons like nitric oxide (NO) and nitrogen dioxide (NO2), the octet rule is not satisfied for all the atoms
• Xenon and Krypton form compounds with elements like fluorine and oxygen, such as XeF2, XeF4, XeO2F4, XeOF4, XeF6, XeO3, XeO4, KrF2
• Inability to predict energy changes during bond formation
• The octet rule does not explain the relative stability of molecules
• The octet rule does not account for the shape of molecules
• Bond length is the equilibrium distance between the nuclei of two bonded atoms in a molecule
• Bond lengths are measured by spectroscopic, x-ray diffraction, and electron-diffraction techniques
• Bond length is usually expressed in Angstrom units (Å) or picometres (pm)
• In an ionic compound, the bond length is the sum of the ionic radii of positive and negative ions in a crystal
• In a covalent bond, the contribution from each atom is called the covalent radius of that atom