Final answer:
The stability of an atom is determined by its number of valence electrons. Atom Z's greater stability suggests it has a full or nearly full valence shell compared to atom Y. Atom Y, with fewer valence electrons, would have higher reactivity.
Step-by-step explanation:
The stability of an atom in chemical reactions is heavily influenced by the number of valence electrons in its valence shell. A full valence shell indicates that an atom is stable, as is the case with the inert gases found in the far right column of the periodic table. Atoms with incomplete valence shells tend to form chemical bonds to achieve stability, following the octet rule, which states that atoms are most stable with eight electrons in their valence shell - a condition that tends to reduce their reactivity.
Without the actual list of valence electrons for atoms Y and Z, we can infer that if atom Z is deemed more stable than atom Y, it likely has a full valence shell, or is closer to having a full valence shell, thereby being less reactive. Meanwhile, atom Y, with a more incomplete valence shell, would be more reactive and form more chemical bonds to achieve stability. The valency of these atoms would depend on how many electrons are needed to complete their valence shells and could further inform their chemical reactivity and bond formation tendencies.