Final answer:
Across a period, metals in groups 1 and 2 increase in reactivity with increasing atomic number, as their ionization energy decreases and they lose electrons more easily. Towards the end of the transition metals, the reactivity trend is less clear due to factors like electron configuration. Across the period, nonmetals increase in reactivity with increasing atomic number.
Step-by-step explanation:
The reactivity of elements within groups labeled A generally changes as the atomic number increases across a period. For metals, especially those in groups 1 and 2 (the alkali and alkaline earth metals), reactivity increases with increasing atomic number due to the ease of losing their valence electrons, which is reflected in the decreasing ionization energy. However, for metals towards the end of the transition series, such as zinc, cadmium, and mercury, this trend does not hold as strongly due to the formation of ions with a pseudo-noble gas configuration and other factors.
In a more generalized approach across the entire period, as you move from left to right across a period, nonmetals increase in reactivity. This is because their increasing nuclear charge more strongly attracts electrons for bonding purposes. Considering atomic radius trends, since the atomic radius decreases from left to right within a period, the outer electrons are closer to the nucleus and thus held more strongly, which influences this reactivity trend.