Chemistry · Chemical Bonding · NEET
Because forming the ions is only step one. When gaseous Na+ and Cl- ions come together and pack into a solid crystal lattice, they attract each other strongly and release a very large amount of energy called lattice enthalpy: -788 kJ/mol for NaCl. Compare: you pay +147.1 kJ/mol to make the free ions, but you get back -788 kJ/mol when they form the solid. The net is about -641 kJ/mol, which is a big energy release. So overall NaCl formation is favourable and the compound is stable.
They are the three energy steps NCERT gives for NaCl. (1) Ionisation enthalpy of Na: Na(g) -> Na+(g) + e-, needs +495.8 kJ/mol (always positive, energy is absorbed). (2) Electron gain enthalpy of Cl: Cl(g) + e- -> Cl-(g), releases -348.7 kJ/mol (energy given out). (3) Lattice enthalpy: Na+(g) + Cl-(g) -> NaCl(s), releases -788 kJ/mol. The sum of the first two is +495.8 - 348.7 = +147.1 kJ/mol, and the lattice step pays that back many times over.
Ionisation enthalpy is always positive because you must always spend energy to pull an electron off an atom. Electron gain enthalpy for Cl is negative (energy released) but its size (348.7) is smaller than the energy spent on ionising Na (495.8). So when you add them, +495.8 and -348.7 give a positive +147.1 kJ/mol. This is normal for many ionic solids and does not stop them from forming.
No. This is the key NCERT point. Na+ and Cl- both have full octets in the gas phase, but that alone does not make the compound stable. NCERT says clearly: the real measure of stability of an ionic compound is its lattice enthalpy, not just achieving the octet. The octet is only reached AFTER the big lattice energy is released. So always think in terms of energy, not just filled shells.
Lattice enthalpy (as enthalpy of lattice formation) is the energy change when gaseous ions come together to form one mole of the solid ionic crystal. For NaCl it is -788 kJ/mol. It is negative because oppositely charged ions attract strongly, and forming these attractions releases energy. The bigger the charges and the smaller the ions, the more negative (larger) the lattice enthalpy, and the more stable the ionic solid.
No. In real life sodium metal and chlorine gas react directly. These steps are just a bookkeeping route (a Born-Haber style path) that lets us add up energies. Because enthalpy is a state function, the total energy change is the same no matter which path we imagine. So we break the real reaction into easy steps whose energy values we know, then add them up.
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Add the three steps: +495.8 (ionise Na) - 348.7 (electron gain Cl) - 788 (lattice) = about -641 kJ/mol. The negative sign means energy is released overall, so NaCl(s) is more stable than the separate gaseous atoms.
Yes. Removing an electron from any atom always needs energy, so ionisation enthalpy is always positive (endothermic). Electron gain enthalpy can be negative or positive.
Lattice enthalpy. NCERT states that the qualitative measure of the stability of an ionic compound is its enthalpy of lattice formation, not simply achieving the octet.
NCERT explains the energetics of NaCl without naming it, but this step-by-step energy path is the Born-Haber idea. NEET can ask it as 'why NaCl forms despite a positive sum', so learn the -788 vs +147.1 comparison.
Because in the solid each ion is surrounded by many oppositely charged ions, and all these coulombic attractions release energy together. Higher ionic charge and smaller ionic size make lattice enthalpy even larger (more negative).
