How is it possible to predict the type of bond that is likely to be found in a substance? what types of atoms generally form ionic or covalent bonds? how can electronegativities be used to determine the type of bond?

Using electronegativity difference is a good guide to the ionic/ covalent nature. Large differences indicate greater ionic character, small differences more covalent character. The larger the difference in electronegativity the more ionic properties a bond is said to have. The smaller the difference in electronegativity the more covalent properties a bond is said to have. 
Ionic bonding is formed through electrostatic attraction between a cation and anion. Foe example, Sodium fluoride has ionic bonding because it is composed by sodium and Fluorine (a non metal). On the other hand, covalent bonding is characterized by atoms sharing pairs of electrons. For example; methane has covalent bonding; carbon has 4 valence electrons and hydrogen has 1; when they bond they have a total of 8 electrons and satisfies the octet rule. 

They can measure  the amount of energy gained or lost