Final answer:
Non-metal reactivity increases across a period due to higher electronegativity and decreases down a group due to increasing atomic size and lower electron affinity. Elements like fluorine are highly reactive due to favorable electron affinity and a high ionization energy.
Step-by-step explanation:
The chemical reactivity of non-metals can be predicted by understanding the periodic trends in atom size, effective nuclear charge, ionization energies, and electron affinities. Within a period, as we move from left to right, non-metals tend to have higher electronegativities and therefore a greater ability to attract electrons. This results in an increase in reactivity as non-metal atoms, such as halogens, aim to achieve the valence electron configuration of the nearest noble gas, predominantly by gaining electrons.
Within a group, the reactivity of non-metals usually decreases as we move down. This is because the outer electrons are farther from the nucleus, making them less tightly held and less effective at attracting additional electrons. Moreover, the increase in atomic size makes it energetically less favorable to gain electrons. However, elements like fluorine have an energetically favorable electron affinity (EA) and a high energy barrier to ionization (IE), making them highly reactive and favoring the formation of anions over cations.
Thus, the general trend is that non-metallic reactivity increases across a period due to rising electronegativities and decreases down a group due to increasing atomic size and decreasing electronegativity.