Final answer:
The correct chemical formula for water is H2O, and it contains 3 atoms. Other molecules like CO2 and glucose (C6H12O6) have different compositions and are products of chemical reactions that must be balanced according to the law of conservation of mass.
Step-by-step explanation:
The correct chemical formula for water is H2O. The other options represent different compounds: C6H12O6 is glucose, a hexose monosaccharide; O2 is molecular oxygen; and CO2 is carbon dioxide. Each of these molecules comprises a certain number of atoms. For example, in H2O, there are 2 hydrogen atoms and 1 oxygen atom, totaling 3 atoms.
Molecules like CO2 and H2O are formed from chemical reactions, such as the combustion of ethane (C2H6) in oxygen (O2), which yields CO2 and H2O as products. It is important to correctly balance chemical equations involving these substances to account for the conservation of mass, as represented by the number of atoms of each element on both sides of the equation.