An atom's valence electron shell Multiple Choice1. is filled when it has three electrons. 2. determines its chemical reactivity. 3. determines its atomic mass. 4. is filled with positively charged particles. 5. is filled identically for every element.

Answer: An atom valence electron shell determines its chemical reactivity.

Explanation:

An atom's valence electron shell determines its chemical reactivity. The amount of electrons in the outermost shell of an atom is its valence electron and it determines how reactive the atom is.

The reactivity of an atom depends on the number of electrons in its outermost shell. Atoms that has their outermost electrons full e. g noble gases like argon, krypton etc are unreactive because there is no room for the atom to donate or accept any electron.

Elements like sodium and chlorine are reactive because they possesses 1 and 7 electrons in their outermost shell respectively as such they can donate and accept electrons making them reactive elements.