Here's why:

* Electrons are located in the outer shells of the atom and are held relatively loosely by the electrostatic attraction of the positively charged nucleus.

* Protons and neutrons reside in the nucleus, which is very tightly bound. Removing or adding protons changes the element itself, while changing the number of neutrons results in isotopes of the same element. These nuclear changes require a significant amount of energy and often involve radioactive processes.

Therefore, gaining or losing electrons is the primary way atoms interact with each other, leading to the formation of ions and chemical bonds.