Before I discuss the actual process whereby a molecule can be changed into an atom under specific, and somewhat difficult to achieve laboratory conditions, it should not go unsaid that a single unit of each element as found in nature can be considered to be a molecule. For example, oxygen is not found in the stable free state as a single atom but as 02, as two oxygen atoms. Technically then, a stable unit, or single molecule of any of the noble gases is already an atom, because the noble gases are monatomic. Anyway, this preamble is more of a digression than an attempt to address the challenge given by the title of this debate, because we all believe, more accurately, that the unqualified definition of a molecule is that it is any single unit of a compound comprised of one or more atoms.
Since molecules are comprised of one or more atoms, one can readily draw the conclusion that a molecule cannot be changed into an atom and that it is only possible to break up a molecule into its constituent atoms, which is not the same thing. However, there is recent experimental evidence that in a roundabout way it can indeed be considered a possible feat to change a molecule into an atom.
There are two major categories of subatomic particles. They are bosons and fermions. Bosons are force-carrying particles, while fermions are matter particles. In 2001, the first Bose-Einstein condensate (BEC) was created out of bosons by cooling them to within a few millionths of a degree Kelvin above absolute zero, which is the state at which all atomic activity ceases. Because atoms by themselves are not bosons they cannot by themselves enter into the Bose-Einstein condensate state. However, Cheng Chin at the University of Chicago and a group at Innsbruck University found that by pairing fermions, namely lithium atoms, thereby creating a new lithium molecule, they then assumed the characteristics of a boson and that they could then be coaxed into a Bose-Einstein condensate.
The researchers used a technique called Feshbach resonance to bind two lithium atoms into a simple molecule that behaves like a boson. Under the application of a magnetic field the atoms paired up in the same way that electrons pair up when they superconduct. This type of bonding is called Cooper pairing. The scientists used a magnetic field in the condensate state to coerce the Cooper-pairing of lithium atoms, in effect meeting the definition of what is considered to be a molecule.
Now then, a characteristic of the Bose-Einstein condensate is that the collection of lithium molecules, or the particles in any other Bose-Einstein condensate for that matter, all assume the same quantum state – termed the ground state – wherein all the atoms behaved in concert. The wave functions of the lithium molecules that make up the Bose-Einstein Condensate overlap. The molecules are said to collapse into what physicists call a “super atom”, which emulates all the behavioral characteristics of a single atom with respect to its de Broglie wave functions. Under BEC conditions, the condensate cloud is for all intents and purposes a single atom.
So you see, there is a way to change a “molecule” into an “atom”.