Chemistry · Chemical Bonding · NEET
NCERT lists them clearly. (1) The metal atom should have LOW ionization enthalpy, so it gives away an electron easily to become a cation (M -> M+ + e-). (2) The non-metal atom should have a HIGH negative electron gain enthalpy, so it accepts an electron easily and releases energy to become an anion (X + e- -> X-). (3) The ions should form a stable crystal lattice that releases a large amount of lattice enthalpy. All three make the ionic compound stable.
Ionization is always endothermic, meaning it always COSTS energy to pull an electron out of an atom. If the metal has low ionization enthalpy, this cost is small. So it is cheap to make the cation. Metals like Na, K, Ca have low ionization enthalpy, which is why they form ionic compounds easily. That is why cations usually come from metallic elements.
You want it to be a high NEGATIVE value, because a negative electron gain enthalpy means energy is RELEASED when the atom accepts an electron. This release helps pay back the energy spent on ionization. Non-metals like Cl, F, O have high negative electron gain enthalpy, so anions usually come from non-metallic elements. Note: electron gain can be exothermic (negative) or endothermic (positive); the more negative it is, the more it favours the ionic bond.
Lattice enthalpy is the deciding factor. Even when the ionization enthalpy plus electron gain enthalpy adds up to a POSITIVE number (energy needed), the crystal can still form because the arrangement of cations and anions in the lattice releases a huge amount of energy. This lattice energy stabilizes the solid. So a high lattice enthalpy is the strongest favourable condition.
Metals have low ionization enthalpy, so they lose electrons easily and become positive cations. Non-metals have high negative electron gain enthalpy, so they gain electrons easily and become negative anions. This is why most ionic compounds have a metal cation and a non-metal anion. The ammonium ion NH4+ is the famous exception: it is a cation made of two non-metals.
Normally cations come from metals, but NH4+ (made from nitrogen and hydrogen, both non-metals) is an exception. It acts as the positive ion in many ionic compounds like NH4Cl. NEET likes to test this exception, so remember: NH4+ is a non-metal-only cation.
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Lattice enthalpy is more important. Even if ionization enthalpy plus electron gain enthalpy gives a positive (unfavourable) sum, a large lattice enthalpy can still make the ionic crystal stable, as happens in NaCl.
Three steps: M(g) -> M+(g) + e- (ionization enthalpy, always endothermic); X(g) + e- -> X-(g) (electron gain enthalpy); and M+(g) + X-(g) -> MX(s) (lattice enthalpy, releases energy).
Yes. The sum of ionization enthalpy and electron gain enthalpy can be positive, but the large energy released when ions pack into a crystal lattice makes the overall solid stable. This is why NaCl forms.
Because it contains the ammonium ion NH4+, which acts as the cation, and Cl- as the anion. It is the well-known exception where a cation is made only from non-metals.
