Lesson 2 of 12 · 9 min
Ionic bonding and lattice enthalpy
NCERT § "Ionic or Electrovalent Bond"
Pulling the electron off a sodium atom costs 496 kJ/mol, and chlorine pays back only 349 kJ/mol when it takes it. On those numbers, salt should never form. Yet the salt jar is full.
The lesson in notes
In short
An ionic bond forms more easily when one atom has a low ionisation enthalpy and the other has a large negative electron gain enthalpy.
Forming isolated gaseous ions usually costs energy; the overall process becomes favourable because of the large energy released when the ions pack into a crystal lattice.
Lattice enthalpy is the energy required to break one mole of a solid ionic compound fully apart into its gaseous ions; for NaCl it is 788 kJ mol⁻¹.
The larger the lattice enthalpy, the more stable the ionic compound.
Ionic solids are crystalline arrays with no discrete molecules, so 'NaCl' describes the ratio of ions, not a molecule.
Electrovalence equals the number of unit charges on the ion formed.