Answer:
D. All of the above
Step-by-step explanation:
The reactivity of alkali metals generally increases with increasing shielding effect, increasing atomic radius, and increasing atomic number.
Increasing shielding effect: The inner electron shells of alkali metals shield the outermost electron from the positive charge of the nucleus. As the number of inner electron shells increases, the shielding effect increases, resulting in decreased attraction between the outermost electron and the nucleus. This lower attraction makes it easier for the outermost electron to be lost during a chemical reaction, increasing reactivity.
Increasing atomic radius: Alkali metals have one valence electron in their outermost shell. As the atomic radius increases (going down the group), the valence electron is further away from the nucleus. This increased distance reduces the attractive force between the valence electron and the nucleus, making it easier for the outermost electron to be lost and increasing reactivity.
Increasing atomic number: Alkali metals have one more proton in their nucleus as you go down the group. The greater positive charge in the nucleus attracts the valence electron more strongly, making it easier for the outermost electron to be lost and increasing reactivity.
Therefore, all of these factors - increasing shielding effect, increasing atomic radius, and increasing atomic number - contribute to the increased reactivity of alkali metals.