Recording-2025-08-22T16:08:58.614Z

Ionic Bonding and the Born Haber Cycle

• Ionic bonding overview: transfer of electrons from one atom to another, followed by electrostatic attraction between resulting ions.

• Sodium and chlorine example: Na gives up an electron; Cl gains an electron to form Na⁺ and Cl⁻.

• Stepwise energy accounting for NaCl formation (gas phase then lattice):

  • Ionization energy of sodium: $I<em>{Na} = 496\ \text{kJ/mol}$. This is the energy required to remove one electron from Na(g): $Na(g) \rightarrow Na^{+}(g) + e^{-} \quad \Delta H = +I</em>{Na} = +496\ \text{kJ/mol}.$

  • First electron affinity of chlorine: $EA<em>{Cl} = 349\ \text{kJ/mol}$ (energy released when Cl(g) gains an electron). The corresponding gas-phase process is: $Cl(g) + e^{-} \rightarrow Cl^{-}(g) \quad \Delta H = -EA</em>{Cl} = -349\ \text{kJ/mol}.$

  • Net gas-phase formation energy for Na⁺ and Cl⁻ from elements in their standard states (without lattice):
    $\Delta H<em>{gas} = I</em>{Na} - EA_{Cl} = 496 - 349 = +147\ \text{kJ/mol}.$

  • Lattice energy (formation of solid NaCl from gaseous ions): $\Delta H_{lattice} = -787\ \text{kJ/mol}.$

  • Overall standard formation enthalpy of NaCl(s):
    $\Delta H<em>{f,\,NaCl(s)} = \Delta H</em>{gas} + \Delta H_{lattice} = 147 - 787 = -640\ \text{kJ/mol}.$

• Takeaway: Even though creating ions involves an unfavorable energy cost, the large lattice energy released when ions crystallize drives the formation of an ionic solid.

• Born–Haber cycle: a useful mental model for organizing the steps and energies involved in forming ionic compounds from elements; helps explain why ionic compounds form and what drives their formation. (Not deeply detailed here, but referenced as a helpful framework.)

• Key insight: the formation of ionic compounds typically comes from electron transfer, then strong electrostatic attraction holds the lattice together. In NaCl, Na donates an electron and Cl accepts it; they stick together as ions.

• Covalent bonding vs ionic bonding (contrast):

  • Covalent bonding: sharing of electrons between atoms rather than full transfer; no full ion formation.

  • Ionic bonding: transfer of electrons to form ions that attract electrostatically.

  • In reality, bonds can have both character; even ionic compounds can exhibit some covalent character, but the dominant interaction in NaCl is ionic due to full electron transfer and strong lattice energy.

Covalent Bonding and Bond Formation Energetics

• Covalent bonding is the attractive force arising from sharing electrons between atoms.

• Why sharing occurs: if it were energetically unfavorable to fully transfer electrons (high ionization energy), atoms instead share electrons to achieve more stable configurations.

• High ionization energy tends to favor covalent bonding: if the energy to remove an electron is very large, forming ions is unlikely, so atoms share electrons instead.

• Periodic trends: ionization energy increases as you move up and to the right on the periodic table. The top-right region (nonmetals