Ionic compounds donot exist as single molecules, Explain

An ionic compound is :

Why do ionic compounds exist as solids?

NaCl is an ionic compound but LiCl has some covalent character, explain:

Ionic compounds are good conductors of electricity. Explain.

PbCl_4 is a stable compound but Pbl_4 has no existance. Explain Why?

Nitrogen exists as diatomic molecule and phosphorous acts as tetra atomic molecule. Explain.

(a) What is an atom ? How do atoms usually exist ? (b) What is a molecule? Explain with an example. (c ) What is the difference between the molecule of an element and the molecule of a compound ? Give one example of each.