To tell if HI (Hydrogen iodide or Hydroiodic acid) is ionic or covalent (also called molecular) we look at the Periodic Table that and see that H is non-metal and Br is a non-metal. When we have a non-metal and a non-metal the compound is usually considered covalent.

For HI there will is a larger difference in electronegativity between the non-metal and non-metal. This difference results in an electron(s) being shared unequally with the non-metal having a higher electronegativity having the electron more of the time..
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Helpful Resources

Metals, Non-Metals on the P- Table: https://youtu.be/OoooStZQHdA
Ionic, Covalent, & Polar Covalent: https://youtu.be/OHFGXfWB_r4
Electronegativity for each element: https://en.wikipedia.org/wiki/Electronegativity
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Because we have a combination of a non-metal and non-metal HI (Hydrogen iodide or Hydroiodic acid) is considered an covalent compound.

For more chemistry help, see http://www.Breslyn.org.