The reactivity of an atom arises from

a. the number of electrons in the nucleus.
b. the number of protons in the nucleus.
c. the number of electrons in the valence shell.
d. the atomic mass.

C. The number of electrons in the valence shell

Explanation:

When the valence shells are complete, the atom is unreactive. However, when they are incomplete, they are chemically reactive.

qualitative observation deals with data that can be observed with our senses: sight, smell, touch, taste, and hearing. they do not involve measurements or numbers. for instance, colors, shapes, and textures of objects are all qualitative observations.