1. what is an isomer? how many possible isomers of hexane are there? what are the structural differences between these isomers?

When two compounds have same molecular formula but different structural formula, they are said to be isomers of each other and this phenomenon is called as isomerism. 

Hexane has six isomers.

The isomers of hexane have a difference in their structure due to parent chain length (chain isomerism). They are also differentiated on the basis of straight chain as in n-hexane and branched chain as in remaining five. Structures along with names are shown below.

Copper, silver, aluminum, cobalt, chromium, mercury