Final answer:
The trend of reactivity across a period on the periodic table does not follow the same pattern as it does down groups 1 and 2, due to complex variations in electron affinity, ionization energy, and atomic size.
Step-by-step explanation:
The question asks about the trend of reactivity across a period on the periodic table, referring specifically to elements in groups 1 and 2 where reactivity increases down the group. The increase in reactivity within groups 1 and 2 is connected to the atomic number. However, across a period, the reactivity trend does not follow the same pattern. This is because reactivity is influenced by various factors such as electron affinity, ionization energy, and atomic size - which can vary in complex ways across a period.
For example, the metallic character of the elements typically increases down a group, as seen with Group 16 where oxygen is a gas, selenium is a semiconducting solid, and polonium is a conductive solid. Additionally, the formation of ions with a pseudo-noble gas configuration can play a role in decreasing reactivity as one moves across the periodic table. For instance, zinc, cadmium, and mercury have different reactivities due to these factors.
An activity series can be used to predict the reactivity of metals when replacing cations in reactions. However, for other elements, trends may not be as straightforward, and the overall reactivity varies based on multiple variables.