What do valence electrons determine in an atom?

Valence electrons play a crucial role in determining an atom's chemical reactivity. These are the electrons located in the outermost shell of an atom, and they are primarily responsible for the formation of bonds with other atoms. The number and arrangement of valence electrons dictate how an atom interacts with others, including whether it will gain, lose, or share electrons during chemical reactions.

For example, atoms with a full outer shell (commonly referred to as noble gases) are typically inert, meaning they do not readily react with other elements. In contrast, atoms lacking a full set of valence electrons tend to be more reactive, as they seek to achieve stability by engaging in chemical reactions. This principle is fundamental to understanding the periodic trends in chemical behavior and the formation of compounds.

In contrast, the other options relate to different properties influenced by other factors. The mass of an atom is determined by its protons and neutrons, while the ability to conduct electricity involves the movement of charged particles, which may include free electrons, not just valence electrons. The strength of the atomic nucleus pertains to the forces that hold protons and neutrons together, independent of the electrons.