Final answer:
Oxygen (O) requires two additional electrons to fill the outermost electron shell.
Step-by-step explanation:
The outermost electron shell, also known as the valence shell, is the electron shell furthest from the nucleus of an atom. The electrons in the outermost shell are called valence electrons, and they play a crucial role in determining the chemical properties and behavior of an element.
The element that requires two additional electrons to fill the outermost electron shell is oxygen (O).