The correct answer is option 3: 8. Valence electrons are the electrons found in the outermost energy level of an atom and they play a crucial role in chemical bonding. In the case of the O2- ion, it is formed when oxygen gains two electrons to achieve a stable electron configuration.

Oxygen has 6 electrons in its outermost energy level (valence electrons) in its neutral state. When it gains two electrons, it will have a total of 8 valence electrons. This full outer shell gives O2- the same electron configuration as the noble gas neon (Ne), which is a stable configuration.

We can determine the number of valence electrons for an atom or ion by looking at its position in the periodic table. In the case of oxygen (O), it is found in Group 16, which means it has 6 valence electrons. When it gains 2 electrons to form the O2- ion, it will have a total of 8 valence electrons.

Therefore, the number of valence electrons in the O2- ion is 8.