Firstly what you need to know is that metallic bonds form in metallic substances. The elements that form these bonds are found on the left of the periodic table. So the metallic bonds form when the charge when the charge is spread over a wide distance when comparing to the size of the atom. These bonds occur between metals like zinc, and since they are in a form of solids, their atoms are tightly packed together.
The formation of metallic bonds occur because the metals exchange electrons, this electron exchange occur in different spatial directions.