If an atom's outer electron shell is filled, what is the most likely characteristic?

• A. It is a stable atom
• B. It is highly reactive
• C. It has a low atomic number
• D. It will readily gain or lose electrons

Answer: (A)

Having a filled outer electron shell means the atom has a complete valence shell, which is a very stable arrangement. When the valence shell is full, there’s little drive for the atom to gain or lose electrons because doing so would disrupt that stability and require energy. That’s why such atoms tend to be relatively nonreactive or inert.

In contrast, atoms with incomplete outer shells tend to react to achieve a full shell—by gaining, losing, or sharing electrons—so they’re typically more reactive. Saying the atom has a low atomic number isn’t a defining indicator of this stability, since filled shells occur at various points on the periodic table. And claiming it will readily gain or lose electrons contradicts the stability of a complete shell.