A property where an atom's outer electron shell is full describes it as

• A. Isotopes
• B. Ionic bond
• C. Chemical formula
• D. Chemically stable

Answer: (D)

Having a full outer electron shell gives an atom a particularly stable electron arrangement. When the valence shell is complete, the atom sits in a low-energy, stable state, which means it is chemically stable and unlikely to react unless forced to change its arrangement.

Isotopes describe atoms with the same number of protons but different numbers of neutrons, so they’re about the nucleus, not the electron shell. An ionic bond refers to a type of chemical bond formed by transferring electrons between atoms, not to the stability of a single atom’s outer shell. A chemical formula is just a way to write which atoms are present in a substance and in what ratio, not a description of an atom’s stability.