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Chemistry: Chemical Bonding: Valence Electrons and Chemical Bond

Valence Electrons

  • The outer most shell of any atom is valence shell and the electrons in the outer most shell are present are called as valence electrons.
  • These electrons form bond and it determines the combining capacity or valency of the atom.

Chemical Bond

  • The two atoms of similar or different elements combine and the energy formed becomes less than the sum of the energies of 2 different atoms at a large distance.
  • The act of combining two atoms is called as chemical bond
  • The chemical bond is an effect that reduce in energy and leads to the formation of a molecule that has distinct properties more than the atoms.
  • The noble gas has stable electronic configuration with 8 electrons in their outermost shell, so they do not react with other elements to form compounds.
  • The stable electronic configuration of noble gases is achieved by many ways they are:
    • By losing an electron
    • By gaining an electron
    • By sharing an electron
  • The chemical bonds are of different types they are Ionic or electrovalent bond, covalent bond or co-ordinate covalent bond and hydrogen bond.

Ionic or Electrovalent Bond

In ionic bond, the atoms acquire the noble gas electronic configuration by gaining or losing electrons.

According to Kossel՚s theory for the formation of NaCl:

  • The electronic configuration of sodium is 2,8, 1 and its highly electropositive in nature. It loses an electron to attain the stable configuration to nearest noble gas atom.
  • Na (2,8, 1) Na+ (2,8) + e-, The enthalpy changes is 493.8 kJ mol-1
  • Cl becomes negative charge and it requires one electron to acquire stable configuration, so, one electron is transferred from sodium to chlorine atoms. Cl + e- Cl-
  • They both are held by electrostatic attraction.
  • Electrovalent or ionic bond is electrostatic attraction that held both anion and cation together.

Energetics of Ionic Compound

Born Harber Cycle (Based on Law of Conservation of Energy)

The formation of NaCl from Na and Cl are shown in simple steps:

Step 1: Conversion from solid to gaseous (it is called as sublimation)

Na (s) Na (g) ; Enthalpy changes = 108.7 kJ mol-1

Step 2: The gaseous sodium atom ionizes to give sodium ion

Na (g) Na+ (g) + e-; Enthalpy changes = 493.8 kJ mol-1

Step 3: Dissociation of gaseous chlorine molecule into chlorine atoms

Cl2 (g) Cl (g) ; Enthalpy changes = 120.9 kJ mol-1

Step 4: The addition of electron converts gaseous chlorine to chloride ion

Cl (g) + e- Cl- (g) ; Enthalpy changes =-379.5 kJ mol-1

Step 5: Finally, NaCl is formed from sodium and chloride ions (Crystal or lattice formation)

Na+ (g) + Cl- (g) Na+ Cl- (s) ; Enthalpy changes =-754.8 kJ mol-1

  • (The energy released in this process is lattice energy)
  • The overall energy change can be computed by sum of all energy changes:

= 180.7 + 493.8 + 120.9 – 379.5 – 754.8 =- 410.9 kJ mol-1

Illustration: Born Harber Cycle (Based on Law of Conservation of Energy)
  • The formation of an ionic compound is determined by ionization energy, electron affinity and lattice energy.
  • In NaCl formation, sublimation and dissociation energies have low energy values than compared to other energies

The formation of ionic compound occurs by:

  • Low ionization energy of the metal
  • High electron affinity of another non-metal
  • High lattice energy

Characteristic Properties of Ionic Compounds

  • The ions in crystalline solids are arranged in a regular 3-D structure
  • The ionic compounds are brittle in nature.
  • The high melting and boiling points in ionic compounds are due to the strong electrostatic interaction between the ions.
  • They are water soluble in nature
  • They are less soluble in non-polar solvents like ether, alcohol etc.
  • They conduct electricity in molten state or in aqueous solutions.