Explain why electrons are shared equally in oxygen, but not in carbon monoxide.?

This is because electrons are more attracted to different types of atoms. This property is called the "electron affinity" of an atom, and it describes how hard a certain atom such as C, or O pulls on an electron. If two atoms are sharing an electron, they each pull on it. In oxygen, the 2 atoms pulling on the electron are O and O so they pull on the electron (and thus share it) equally. In carbon monoxide, the 2 atoms are C and O. O pulls on the electron harder than the C does, so like in a tug-of-war, the electron leans slightly towards the O atom.