Final answer:
An atom's reactivity is highest in Group 1, composed of alkali metals with a single electron in their outer shells, and lowest in Group 18, the noble gases with filled outer shells. Reactivity is also influenced by electronegativity, where lower values correlate with higher reactivity in these groups.
Step-by-step explanation:
The student's question is seeking to understand which groups in the periodic table feature elements with the highest and lowest reactivity, respectively. An atom's likelihood of reacting with other atoms is highest in Group 1 and lowest in Group 18.
Group 1, which includes alkali metals like lithium (Li), sodium (Na), and potassium (K), is characterized by elements that have a single electron in their outer shell. This makes them highly reactive, as they can easily lose this electron during chemical reactions. On the other hand, the Group 18 elements, also known as noble gases (helium, neon, argon, etc.), have a completely filled outermost subshell, which makes them stable and unreactive under normal conditions.
Electronegativity plays a role in defining reactivity as well. It is a measure of an atom's tendency to attract electrons. Group 1 elements have low electronegativity, while Group 17 elements (halogens) have high electronegativity but are not as unreactive as noble gases due to their tendency to gain an electron to achieve stability.