What do valence electrons determine about an atom?

Valence electrons are the electrons located in the outermost shell of an atom and play a crucial role in determining how an atom interacts with others. These electrons are the ones that are involved in chemical bonding and reactions, influencing an atom's reactivity, the types of bonds it can form (ionic, covalent, etc.), and its stability. The arrangement of these electrons dictates the atom's ability to gain, lose, or share electrons during chemical interactions, thus defining its chemical behavior.

The other options focus on different aspects of an atom that are not directly influenced by valence electrons. For instance, the mass of an atom is primarily determined by the number of protons and neutrons in its nucleus. The position of an atom in the periodic table is largely based on its atomic number (number of protons) and electron configuration, while the temperature at which it reacts is influenced by various external factors like energy levels and conditions, rather than solely by the valence electrons themselves. Therefore, the most relevant aspect that valence electrons define is the chemical behavior of the atom.