Atoms are unstable unless they have a similar number of protons and neutrons. To make the carbon atom stable, add six neutrons to it. What’s the mass number now?

a polar bond is formed when electrons are shared unevenly between atoms

chemical changes occur when a substance combines with another to form a new substance, called chemical synthesis or, alternatively, chemical decomposition into two or more different substances. these processes are called chemical reactions and, in general, are not reversible except by further chemical reactions. some reactions produce heat and are called exothermic reactions and others may require heat to enable the reaction to occur, which are called endothermic reactions. understanding chemical changes is a major part of the science of chemistry.