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water molecules are composed of atoms (2 Hydrogens and 1 Oxygen) .. so an atom cannot be water
Hydrogen and oxygen are elements. What that really means is that when an atom, the fundamental unit that makes up everything, has 1 proton (a positively charged sub-atomic particle) it is a hydrogen atom, and when it has 8 protons, it is an oxygen atom. Hydrogen gas is two hydrogen atoms bonded together, H2. Oxygen gas is two oxygen atoms bonded together, O2. Water, is one oxygen atom bonded to two different hydrogen atoms, H2O. When atoms are bonded together, (H2, O2, H2O etc.) we call the total structure a molecule. Hydrogen atoms and oxygen atoms are found in many different molecules, and these different molecules make up, solids, liquids, and gasses. Whether a material is a solid, liquid, or gas depends on how these molecules (made up of atoms) interact with each other. If there is strong interaction the molecules like to stick close together, and they are solids. In liquids the interactions are a little less strong, and in gases, the molecules are not attracted to each other as much at all.
"an atom, the fundamental unit that makes up everything" you haven't heard of particle physics eh :P