Chemistry · Chemical Bonding · NEET
NCERT says: ionic bonds form more easily between elements with comparatively LOW ionization enthalpies and elements with comparatively HIGH NEGATIVE electron gain enthalpy. In plain words: the metal should give up its electron cheaply, and the non-metal should release a lot of energy when it takes that electron. That is why metals (left side) pair with non-metals (right side).
Ionization enthalpy is the energy you must SPEND to pull an electron off a gas-phase atom: M(g) to M+(g) + e-. This step is always endothermic (costs energy). If the metal has low ionization enthalpy, making the cation costs little energy, so the whole ionic bond is easier to form. Group 1 and 2 metals (Na, K, Ca) have low values, so they form cations readily.
For the non-metal you want electron gain enthalpy to be a large NEGATIVE number. Electron gain enthalpy is the energy change when a gas-phase atom gains an electron: X(g) + e- to X-(g). A negative value means energy is RELEASED (exothermic) and the anion is stable. Halogens like Cl and F have highly negative electron gain enthalpy, so they form anions easily. 'High negative value' = strongly favourable.
Ionization enthalpy is about REMOVING an electron (making a cation) and is always endothermic (needs energy). Electron gain enthalpy is about ADDING an electron (making an anion) and can be exothermic (usually) or endothermic. One deals with the metal side, the other with the non-metal side. Both must be favourable for an easy ionic bond.
Pulling an electron away from a positive nucleus always fights an attraction, so you must always add energy — ionization is always endothermic. Adding an electron often releases energy because the incoming electron is pulled in by the nucleus, so electron gain is usually exothermic. But for some atoms (like noble gases, or forming a 2- ion) it can be endothermic.
No. Low ionization enthalpy and high negative electron gain enthalpy only make the two IONS form easily. The compound actually forms because of the third factor: lattice enthalpy — the strong attraction that packs the cations and anions into a solid crystal and releases a large amount of energy. For NEET, remember: ion formation (these two enthalpies) PLUS lattice enthalpy together decide it.
They are closely related but opposite in sign in NCERT usage. NCERT states that electron affinity is the NEGATIVE of the energy change accompanying electron gain. So a strongly exothermic (very negative) electron gain enthalpy corresponds to a large positive electron affinity. In NEET questions, stick to 'high negative electron gain enthalpy favours anion formation.'
Try the real previous-year questions from this chapter — each with the answer and a full solution.
1) The metal atom should have low ionization enthalpy so its cation forms easily. 2) The non-metal atom should have a high negative electron gain enthalpy so its anion forms easily and releases energy.
Alkali and alkaline earth metals (Na, K, Ca — low ionization enthalpy) with halogens and oxygen (Cl, F, O — high negative electron gain enthalpy). That is why NaCl, KCl and CaO are typical ionic compounds.
Yes, it is the NCERT exception. NH4+ is a cation made of two non-metals (N and H), yet it forms many ionic compounds like NH4Cl. So not every cation comes from a metal.
Because the large negative lattice enthalpy (energy released when ions pack into the crystal) more than pays back the cost. This is shown by the Born-Haber cycle for NaCl.
Lattice enthalpy. Ionization and electron gain enthalpy make the ions; lattice enthalpy holds them together and usually decides whether the solid is stable overall.
