PRELIMS

Chemical Bonding

Chemical bonding is the force that holds together atoms in a molecule.

Ionic or Electrovalent Bond

• Definition: Bond formed by the complete transfer of one or more electrons from one atom to another. (Example: formation).
• Conditions:
  1. Ionization Energy of the metal should be low.
  2. Electron Affinity of the non-metal should be high.
• Properties of Ionic Compounds:
  1. High melting point and boiling point.
  2. Good conductors in molten state or solution (dissolved in water).
  3. Bad conductors in solid state (ions are not free).
  4. Soluble in water; insoluble in non-polar solvents (Benzene).

Covalent Bond

• Definition: Bond formed by mutual contribution and sharing of electrons (e.g., ).
• Properties of Covalent Compounds:
  1. Low MP/BP compared to ionic (usually).
  2. Bad conductors of electricity (Exception: Graphite).
  3. Insoluble in water; soluble in organic solvents.
  4. Covalent bonds are directional.

Coordinate Bond (Dative Bond)

• Definition: Special type of covalent bond where one atom (donor) provides both electrons to another atom (acceptor). (Example: ).
• Notation: Denoted by an arrow .

Sigma ( ) and Pi ( ) Bonds

• Sigma Bond: Formed by linear overlapping of atomic orbitals. Strong bond (large overlapping).
• Pi Bond: Formed by sidewise (lateral) overlapping. Weak bond (lesser overlapping).
• Hierarchy: bond is stronger than bond.

Bond Energy and Bond Length

• Bond Energy: Energy required to break one mole of bonds in gaseous state.
  • Factors: Atom size (inverse relationship) and Multiplicity (direct relationship).
  • Energy order: Single bond < Double bond < Triple bond.
• Bond Length: Equilibrium distance between nuclear centers.
  • Factors: Atom size (direct) and Multiplicity (inverse).

Metallic Bond

• Definition: Force of attraction between positively charged metal ions (kernels) arranged in a lattice and a surrounding "sea" of delocalized, mobile valence electrons.
• Properties Explained: Good electrical/thermal conductivity (free electrons carry charge/heat), malleability and ductility (layers of ions can slide without breaking the bond), lustre (free electrons reflect light).
• Strength: Depends on number of valence electrons and size of metal ion; generally increases across a period among metals.

Hydrogen Bond

Hydrogen between two highly electronegative atoms (N, O, F) forms a covalent bond with one and a weak attraction (Hydrogen bond) with the other. Denoted by dotted lines .

1. Intermolecular: Bond between different molecules. (e.g., Water, Alcohol).
   • Effect: Increases MP/BP due to molecular association. Alcohol is soluble in water due to this.
2. Intramolecular: Bond within the same molecule.
   • Effect: Decreases MP/BP due to cyclisation.

UPSC Relevance

Understanding bond types is critical for materials science and biochemistry.

• Graphite Exception: Why a covalent solid can be a good conductor.
• Hydrogen Bonding: Explains physical unique properties like water's high boiling point compared to similar hydrides.
• Solubility Rules: Why oil (non-polar) won't mix with water (polar) based on bond types.