Ionic bond and ionic radii in FeS
Iron and sulfur ions bond by electrostatic attraction. 
  


This is the right answer. I see a hard time maneuvering around it.


  
  Breaking an ionic bond between $\ce{Fe^2+}$ and $\ce{S^2-}$ releases energy.
  


Around the basic definition of forces and bonds, there are usually two big beliefs. One is a big misconception, and the other one is right.

The misconception: Some students fall in the trap of believing that bonds are like boxes of energy. So, breaking these boxes of energy should release energy, right? No! In reality, ionic bonds are nothing but forces. This is like imagining that when you pull a box, you're applying or somehow putting some kinda a box.

The reality: Ionic bonds are just forces that are a result of attraction between two species (such as two atoms). Why do these happen in the first place? Because for instance in \ce{NaCl} the sodium atom really likes to lose an electron and chlorine wants to earn one. This love of giving and/or taking an electron is because of reaching the stability. Stability in this case means lesser energy. So, ionic bonds happen because atoms want to have lesser energy. 

Finally, what would breaking this bond result in? If the atoms bonded together to lower their energies, then what would removing the bond result in?


  I don't think the ionic radius of an iron ion is equal to its atomic radius.


You're right! Think of the sodium chloride example again. The new sodium cation used to have 11 protons and 11 electrons. Now, after the lattice builds up, sodium crudely has only 10 electrons, with the same 11 protons. The electrons are outnumbered! They can't overpower protons anymore. So, what will happen to the distance electrons were able to travel to? (The distance they could "travel" further from the nucleus was called the atomic radius, and it's the ionic radius now)
So, is the atomic radius of iron equal to its ionic radius?



Rather pedantic and irrelevant, but there's no clear distinction between an ionic bond and a covalent bond. There are no "pure" ionic bonds.