What family of KBOs does Orcus belong to?

Pluto

Explanation:

Orcus is a plutino, a trans-Neptunian object that is locked in a 2:3 resonance with the ice giant Neptune, making two revolutions around the sun to every three of Neptunes. This is much as pluto, except that the phase of Orcus's orbit is opposite from Pluto's: Orcus is at aphelion when pluto is at aphelion when Pluto is at perihelion and vice versa. This was a major consideration in selecting its name, as deity Orcus was the Etruscan equivalent of the Roman Pluto, and later becamean alternate name for Pluto

when atoms of different types of elements join together, they make molecules called compounds. water consists of compound molecules made up of 2 hydrogen atoms and 1 oxygen atom. this is why it's called h2o. water will always have 2 times the number of hydrogen atoms as oxygen atoms.