Final answer:
C. Alkali metals, which are part of Group 1 on the periodic table, have 1 valence electron. They are highly reactive and commonly form positive ions by losing their single, loosely bound valence electron.
Step-by-step explanation:
The element that has 1 valence electron is the alkali metals. Alkali metals belong to Group 1 of the periodic table and include elements like lithium, sodium, and potassium. They are known for having a single s electron in their outermost subshell (an s¹ configuration), which makes them highly reactive chemically. Alkali metals readily give up their loosely bound outermost electron, thus they tend to form singly positive ions like Na⁺. For instance, lithium is a light alkali metal with three protons and is the Group 1 element with the lowest atomic mass.
Elements in any one group (or column) have the same number of valence electrons; the alkali metals lithium and sodium each have only one valence electron, the alkaline earth metals beryllium and magnesium each have two, and the halogens fluorine and chlorine each have seven valence electrons. The similarity in chemical properties among elements of the same group occurs because they have the same number of valence electrons. It is the loss, gain, or sharing of valence electrons that defines how elements react.