Final answer:
An element's reactivity is mainly determined by its number of valence electrons, with incomplete valence shells leading to higher reactivity. Noble gases are unreactive due to their full valence shells. The periodic table, interaction with other elements, and temperature are also important factors in an element's reactivity.
Step-by-step explanation:
The reactivity of an element is determined primarily by the number of electrons in its valence shell, which is the outermost shell of an atom. When this shell is not full, the atom is more reactive because it tends to interact with other atoms to achieve a full valence shell, thus achieving a more stable configuration. Elements like hydrogen, with only one electron in their valence shell, are highly reactive because they seek to complete their valence shells either by losing, gaining, or sharing electrons.
Noble gases, on the other hand, exhibit very low reactivity due to having full valence shells. This stable electron configuration resists adding or removing electrons, making these elements less likely to participate in chemical reactions. The periodic table can be used to predict the behavior and reactivity of elements based on their group and the number of valence electrons.
Factors such as the formation of halide compounds, the displacement of hydrogen from dilute acids, and the energy required to remove electrons from an atom also play roles in determining an element's reactivity. Additionally, temperature influences reactivity, as reactions generally occur faster at higher temperatures due to the increased kinetic energy of the particles involved.