Which of the following best describes valence electrons?

Valence electrons are best described as the electrons involved in chemical bonding. These electrons are located in the outermost shell of an atom and play a crucial role in determining how atoms interact with one another. The number of valence electrons affects an atom's reactivity, ionization energy, and electronegativity, making them fundamental in the formation of chemical bonds, whether ionic or covalent.

In contrast, electrons in the nucleus do not contribute to chemical bonding since they are contained within the atomic nucleus, forming part of the atom's structure alongside protons. Additionally, the notion of stable and unreactive electrons pertains more to filled electron shells (as seen in noble gases) rather than a definition of valence electrons themselves. Lastly, the mass of an atom is primarily determined by protons and neutrons in the nucleus, not by the valence electrons, which are lighter and located outside the nucleus. Thus, understanding that valence electrons are pivotal in bonding helps clarify their role in the behavior of elements during chemical reactions.