Final answer:
The unknown element with a valence electron configuration of ns²np⁴ is Oxygen (O).
Step-by-step explanation:
The unknown element with a valence electron configuration of ns²np⁴ is a non-metal. To determine the identity of this element, we need to find an element with the electron configuration 1s²2s²2p⁴. Among the given options (Oxygen, Fluorine, Sulfur, Chlorine), only Oxygen (O) has this electron configuration. Oxygen has 6 valence electrons in its second energy level (2s²2p⁴). When it gains two electrons to form the O²- anion, it achieves a stable noble gas electron configuration similar to neon (1s²2s²2p⁶).