When a sodium atom and a chlorine atom exchange an electron, they form an ionic bond. This is because the sodium atom gives up an electron to the chlorine atom, creating a positively charged sodium ion and a negatively charged chloride ion. These oppositely charged ions then attract each other, forming a stable compound.

The exchange of electrons between atoms is what makes chemical reactions possible. When atoms gain or lose electrons, they change their chemical properties and can form new molecules. In the case of sodium and chlorine, the exchange of electrons results in the formation of sodium chloride, which is a common table salt.