Here's why:

* Electronic configuration: Oxygen's atomic number is 8, meaning it has 8 protons and 8 electrons. Its electronic configuration is 1s² 2s² 2p⁴.

* Valence electrons: Valence electrons are the electrons in the outermost shell of an atom, which are involved in chemical bonding. In oxygen's case, the outermost shell is the second shell (2s² 2p⁴), containing a total of 6 electrons (2 from the 2s subshell and 4 from the 2p subshell).

Therefore, oxygen-16 has 6 valence electrons.