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Low ionisation energy
Ionisation energy is the amount of energy, which is required to remove the most loosely bound electron(s) from an isolated gaseous atom to form a positive ion. In forming an ionic bond, one atom must form a cation by losing one or more electrons. In general, elements having low ionisation energies have a more favourable chance to form a cation, thereby having a greater tendency to form ionic bonds. Thus, lower ionization energy of metallic elements favours the formation of an ionic bond. It is because of low ionization energy that the alkali and alkaline earth metals, form ionic compounds.
High electron affinity
Electron affinity is the amount of energy released, when an isolated gaseous atom accepts an electron to form a negative ion. The other atom participating in the formation of an ionic compound must form an anion by gaining an electron (s). Higher electron affinity favours the formation of an anion. Therefore, generally, the elements having higher electron affinity favour the formation of an ionic bond. Halogens have high electron affinities, and therefore halogens generally form ionic compounds.
Large lattice energy
When a cation, and an anion come closer to each other, they get attracted to each other due to the coulombic force of attraction. These electrostatic forces of attraction between oppositely charged ions release a certain amount of energy (when the ions come closer) and an ionic bond is formed. If the coulombic attractions are stronger, then more energy gets released and a more stable ionic bond is formed.
Lattice energy 'is the energy released when one mole of an ionic compound in crystalline form is formed from the constituent ions'. Therefore, larger lattice energy would favour the formation of an ionic bond. Lattice energy thus is a measure of coulombic attraction between the combining ions. The lattice energy of an ionic compound depends directly on the product of the ionic charges, and inversely on the square of the distance between them.Lattice Energy=q1xq2\d2.Thus, small ions having higher ionic charge shall have larger lattice energy. Lattice energies of various sodium halides are:
The minus sign of lattice energy indicates that the energy is released from ions in the gaseous state, during the formation of solid ionic compound.
An ionic bond is formed through the steps described above. Now, if the total energy released is more than that which is absorbed, then the formation of ionic compound is favoured. The conditions that favour the formation of an ionic bond (or ionic compound) are summarized below:- Low ionisation energy of the metallic element, which forms the cation.
- High electron affinity of the non-metallic element, which forms the anion.
- Large lattice energy i.e., the smaller size and higher charge of the ions.
Problem
2. Show whether the formation of ionic bond in sodium chloride is favourable if the ionization potential of Na is 495.8 kJmol-1, the electron affinity of Cl is 349.4 kJmol-1 and lattice energy is 776kJmol-1.
Solution
The formation of ionic bond of sodium chloride can be described through the following steps.
Step 1
Ionization of Na(g) to Na+(g)
In this step, energy equal to the ionisation energy of sodium is absorbed.
Step 2
Formation of chloride ion from chlorine atom in the gaseous phase.
In this step energy equal to the electron affinity of chlorine is released.
Step 3
Formation of solid sodium chloride from Na+(g) and Cl-(g).
In this step, energy equal to the lattice energy is released.
= 776+349.4 kJ= 1125.4 kJ
Total energy absorbed in step (1) = 495.8 kJTherefore,
Net energy released per mole of NaCl formed = 629.6 kJSince, energy (= 629.6 kJ/mol) is released during the formation of sodium chloride for Na(g) and Cl(g), hence the ionic bond in Na+Cl- is stable.
