Take over is a phrasal verb (e.g., “The conqueror wants to take over the country to the east.”) Takeover is a noun (e.g., “The takeover of the country was peaceful.”) and sometimes an adjective. The one-word form does not function as a verb.
Take over is one of countless phrasal verbs that have corresponding one-word forms that function as nouns and adjectives. The one-word forms never catch on as verbs in edited, published writing, but they are commonly treated as verbs in informal contexts