Here's why:

* Electron Configuration: Oxygen's electron configuration is 1s²2s²2p⁴.

* Valence Shell: The outermost shell (the 2nd shell in this case) is called the valence shell.

* Valence Electrons: The electrons in the valence shell are called valence electrons. In oxygen's case, there are 2 electrons in the 2s subshell and 4 electrons in the 2p subshell, giving a total of 6 valence electrons.