Final answer:
Electronegativity (EN) determines the type of bond formed between atoms: nonpolar covalent for small differences in EN, polar covalent for moderate differences, and ionic for large differences. Bond formation is exothermic, releasing energy, while bond breaking is endothermic, requiring energy input.
Step-by-step explanation:
The role of electronegativity (EN) in bond formation is crucial in determining the type of bond that will form between two atoms. When the difference in EN is small or zero, atoms share electrons equally, resulting in a nonpolar covalent bond. If the EN difference is moderate, like 1.0, electrons are shared unequally, creating a polar covalent bond, evidenced by molecules such as CO where oxygen's higher EN draws electrons closer. Lastly, when the EN difference is greater than 1.8, the sharing is so unequal that it leads to an ionic bond where electrons are transferred from one atom to another.
Bond formation is an exothermic process, releasing energy in the form of heat, while bond breaking is endothermic, absorbing the same amount of energy. Understanding bond energy and EN enables predictions about the behavior of molecules during chemical reactions, whether they will be exothermic or endothermic.