The molecules of which of the following substances contain the same kind of…

A pure substance is classified, in its molecular form, as an element when every atom in its molecule is of the same kind; it is classified as a compound when its molecules contain atoms of two or more different elements bonded together.

• Oxygen is O2, so every atom in its molecule is an oxygen atom — the same kind throughout.

• Water is H2O, combining hydrogen atoms with an oxygen atom, so two different kinds of atoms are present.

• Carbon dioxide is CO2, one carbon atom bonded to two oxygen atoms — again two different kinds.

• Sulphur dioxide is SO2, one sulphur atom bonded to two oxygen atoms — again two different kinds.

Only the formula for oxygen repeats a single element symbol; the other three formulas each combine two distinct element symbols, confirming oxygen is the only substance listed whose molecules contain the same kind of atom throughout.