An atom with a completely filled valence shell is generally considered:

An atom with a completely filled valence shell is considered stable because its outer electron shell is full, typically following the octet rule for heavier elements. This stability minimizes the tendency of the atom to lose, gain, or share electrons, making it less reactive. Atoms with filled valence shells, such as noble gases, are often unreactive due to their lack of tendency to form bonds with other atoms.

In contrast, atoms with partially filled valence shells are more reactive because they will seek to achieve stability by either gaining, losing, or sharing electrons. Thus, stability is a key characteristic of atoms with a full valence shell, emphasizing their inertness and lack of chemical reactivity.