What determines an atom's chemical stability?

An atom's chemical stability is primarily determined by its valence electron configuration. Atoms strive to achieve a full outer shell of electrons, which typically consists of eight electrons, known as the "octet rule." When the outermost shell of electrons is full, the atom tends to be stable and less reactive.

Atoms can achieve this stability through various means, such as sharing electrons in covalent bonds, transferring electrons in ionic bonds, or by simply not engaging in reactions at all, if they already possess a complete set of electrons in their outer shell. For example, noble gases, which have a complete outer electron shell, are very stable and do not react easily with other elements.

While the mass number, nuclear stability, and atomic size are important factors in understanding certain aspects of an atom, they do not directly dictate its chemical reactivity and stability in the same way that the electron configuration at the valence level does. Thus, the valence electron configuration is the key to understanding an atom's chemical stability.