National League for Nursing (NLN PAX) Practice Exam

Which property helps determine the reactivity of an element?

1. The number of protons

2. The number of neutrons

3. The number of valence electrons

4. The element's density

The correct answer is: The number of valence electrons

The reactivity of an element is primarily influenced by its valence electrons, which are the electrons located in the outermost shell of an atom. The number of these valence electrons determines how an element will interact with others during chemical reactions. Elements aim to achieve a stable electron configuration, often resembling that of the nearest noble gas. For example, elements with one or two valence electrons, like alkali metals, are highly reactive as they tend to lose their valence electrons easily to achieve that stability. Conversely, elements with seven valence electrons, such as halogens, tend to be very reactive as they seek to gain an electron to fill their outer shell. In contrast, the number of protons determines the element's identity and position in the periodic table, while the number of neutrons primarily influences the atomic mass and stability of isotopes. Density does not play a direct role in reactivity but rather indicates how closely packed the atoms are within a material. Thus, it is the number of valence electrons that is essential for understanding an element's reactivity in chemical situations.