Final answer:
Valence electrons are responsible for determining an element's chemical reactivity, influencing how it forms chemical bonds and its behavior in reactions.
Step-by-step explanation:
Valence electrons determine the chemical reactivity of an element. These are the outermost electrons of an atom and play a key role in chemical reactions. Elements with the same number of valence electrons often exhibit similar chemical properties because they have similar valence shell electron configurations. The chemical bonding abilities of elements, such as the number and types of bonds they can form, are also determined by these valence electrons.
The number of valence electrons in an element's atom can be predicted by its position in the Periodic Table. For instance, elements in the same group (vertical column) have the same number of valence electrons, which accounts for their similar properties. In contrast, as you move across a period (row), the number of valence electrons gradually changes, altering the elements' properties to some extent.
Therefore, the correct answer to the question 'What do valence electrons determine about an element?' is 'b. Chemical reactivity'.