Oxygen requires two more valence electrons in compounds like water (H2O) because it allows oxygen to achieve a stable octet configuration. Oxygen is in Group 16 of the periodic table, and atoms in this group tend to gain two electrons to achieve a full valence shell, which corresponds to eight electrons.
In the case of water, oxygen forms two covalent bonds with hydrogen atoms. Each hydrogen atom shares one electron with oxygen, resulting in a total of two shared electrons and two lone pairs on oxygen. This arrangement satisfies the octet rule, where oxygen has a total of eight valence electrons (6 from its original six valence electrons plus 2 from the shared electrons).
By gaining two electrons through bonding, oxygen completes its valence shell with a total of eight electrons, which is a stable electron configuration similar to the noble gas configuration of helium (He). Achieving a stable octet allows oxygen to be more energetically favorable and contributes to the stability and reactivity of compounds like water.