Question

what is lattice energy ? I want a proper definition with explanation please

Answer 1

Try this http://chemed.chem.purdue.edu/genchem/topicreview/bp/ch7/lattice.html

Answer 2

Lattice energy is the difference in energy between separated ions and a crystal made of those ions. You might wonder why there IS any difference, and the answer is that when the ions come close to each other, to form the crystal, there are very strong attractions (between the unlike ions, e.g. Na+ and Cl-) and very strong repulsions (between the like ions, e.g. between Na+'s and Cl-'s).

if the lattice is to form at all, the ions need to arrange themselves so that on average the attractions outweigh the repulsions. You can do this by surrounding each cation with anions, and vice versa. For example:

Na+ Cl- Na+ Cl- Na+ Cl-
Cl- Na+ Cl- Na+ Cl- Na+
Na+ Cl- Na+ Cl- Na+ Cl-

Notice that each Na+ is surrounded by a ring of Cl-'s, and each Cl- is surrounded by a ring of Na+'s. The shorter distances between the unlike ions, compared to the distance between the like ions, mean the attractions are stronger than the repulsions. (Remember Coulomb's Law: the force between charges q1 and q2 goes like q1 * q2/ r^2, where r is the distance between them. The force falls off steeply with increasing separation between charges.)

So with a particular arrangement of the ions, it is possible for the attractions to outweight the repulsions, which means the energy of the crystal will be lower than the energy of the separated ions. The exact amount by which it is lower is complicated to calculate, because you need to sum up all the Coulomb force contributions in an infinite lattice. Takes a little math. But if you do figure it out, this difference is the lattice energy.

Lattice energy is important because without it ionic compounds would not be possible. With extremely rare exception, removing electrons from neutral atoms to create cations takes more energy than is released when you allow the removed electrons to attach to other neutral atoms to create anions. That is, this process is endothermic:

Na(s) + 1/2 Cl2(g) = Na+(g) + Cl-(g)

The only reason a compound like NaCl exists is because if you did this, the Na+ cations and Cl- anions could form a crystal with a lattice energy sufficiently large to compensate for the energy absorbed in creating the ions in the first place.