Which concept states that atoms are most stable when their valence shell is filled?

• A. Metalloids
• B. Ionization energy
• C. Covalent bonds
• D. Octet rule

Answer: (D)

The key idea is that atoms gain stability by completing their outermost electron shell. For most main-group elements, having eight electrons in the valence shell (a noble gas configuration) makes the atom energetically the most stable. To reach that full shell, atoms tend to gain, lose, or share electrons through bonding, so they end up with a filled valence shell. Hydrogen and helium are exceptions, being happy with two electrons in their only shell.

That drive to fill the valence shell is what the octet rule describes. It explains why atoms form covalent bonds by sharing electrons or ionic bonds by transferring electrons—both pathways help atoms achieve that stable, filled outer shell. Other terms like metalloids or ionization energy describe different properties or processes, not the principle that stability comes from a filled valence shell.