Question

What is the basic unit of an ionic compound and a covalent compound?

Answer 1

The basic unit of an ionic compound is called a formula unit. The basic unit of a covalent compound is a molecule.

Explanation:

Ionic compounds exist as ionic lattices containing large numbers of positive and negative ions. The formula unit is the lowest whole-number ratio of positive to negative ions.

Halite crystals are composed of huge numbers of sodium and chloride ions arranged in a three-dimensional lattice.
http://www.cod.edu/people/faculty/jarman/richenda/1211_MATERIALS/Type-1.htm

Covalent compounds are composed of individual molecules. An example is the polar molecule water. The symbols `delta^+` and `delta^-` means partially positive and partially negative, respectively. Because of the polarity of water molecules, they do form hydrogen bonds between molecules, but these are not the same as covalent or ionic bonds.
http://www.personal.psu.edu/staff/m/b/mbt102/bisci4online/chemistry/chemistry3.htm