Compounds are called such, because they are composed of more than one element. Water is H2O. 2 Hydrogens, 1 Oxygen.
It can be broken down into simpler substances. Those substances would be the individual elements that make it up. We can do this through a process called hydrolysis.
You NEVER refer to a compound as being in units of “atoms”. EVER! an atom is an individual element. We refer to individual particles of compounds as “molecules”.