What I do not understand is how scientists can tell whether or not an atom is bonded to another atom or all by itself. If someone can help me understand this, please do.

  • $\begingroup$ Keep in mind that it was not until the last third of the twentieth century that anyone was able to talk about experimental operations on a single, particular atom. Up until then (and in most cases since then as well), experiments are about atoms taken together (and usually in significant numbers). So is the question about how scientisist know that a particular chemical substance is atomic rather than molecular or is it more specific than that? $\endgroup$ – dmckee --- ex-moderator kitten Jul 29 '18 at 3:40
  • $\begingroup$ No, nothing more specific than that. $\endgroup$ – Anthony Ducharme Jul 29 '18 at 4:02
  • $\begingroup$ Love this part of the title "experimentally (this means not theoretically)" $\endgroup$ – KF Gauss Jul 29 '18 at 5:32
  • $\begingroup$ I would say that the usual way to determine the bonding is to look at: vibrational modes of the atom, optical (IR to EUV) absorption, and xray absorption at specific core levels $\endgroup$ – KF Gauss Jul 29 '18 at 5:35
  • $\begingroup$ @AnthonyDucharme, does my answer answer your question? The wet chemistry of a macroscopic sample is fairly straightforward. The bonded atom assay could make use of the listed theoretical quantum mechanical effect distinctions between bound and unbound atoms. Scanning for absorption and emission in the Infrared, Visible, and UV bands - looking for evidence of the energetic resonance produced by the orbital energetics associated with bonded vs. unbonded electrons seems possibly realistic. The difficult aspect of this assay is evaluating the bond with only a single atom/molecule. $\endgroup$ – Thomas Lee Abshier ND Jul 29 '18 at 17:54

What laboratory tests can be conducted to determine whether an unknown sample (of macroscopic amount) contains a mixture of pure elements or elements bonded into a molecule?

  • Bonding usually absorbs or releases energy, thus experiments may be done on the unknown to measure the amount of energy evolved in response to various reagents.
  • Bonded atoms form compounds/molecules with unique physical characteristics. By measuring the various characteristics of the unknown, we may then compare those measured physical parameters with the physical properties of known samples of pure elements and their compounds.
  • The comparison and matching of measured data with the table of recorded values will reveal whether the unknown was composed of bonded elements or a mixture of pure unbonded elements.

Experiments to perform on the unknown include:

  • 1) Measure the unknown's heat capacity, latent heat of fusion, and heat of vaporization.
  • 2) Dissolve the unknown in various solvents: water, acid, alkali, and non-polar solvents. What is its solubility in each?
  • 3) React the unknown with various reagents and measure the amount of energy released or absorbed and compounds formed.
  • 4) Measure its temperature of freezing and evaporation.
  • 5) Conduct x-ray crystallography measurements - what is the interatomic spacing and packing order of the unknown?
  • 6) Weigh and measure the unknown's displacement to determine its density.
  • 7) Stimulate the unknown with radiation of various frequencies and chart its emission and absorption frequencies, reflectivity, transmissivity, and index of refraction.
  • 8) Allow the unknown to diffuse through a column or strip with and without electrical charge and at different pH's.

But, possibly the question is asking how to determine if two atoms are bonded if the sample contains only 2 atoms. In that case, we must measure the quantum state signatures associated with bonded and unbonded atoms to give evidence of the presence or absence of bonding.

  • When two atoms bond, the bonding process will absorb or evolve energy. Thus, the mass of the molecule will be different than the sum of the mass of the individual unbonded atoms. Detecting the mass defect due to the loss of energy due to bonding energy change may be possible, but I will leave the specifics of this technology to others with more knowledge/experience in such laboratory techniques.
  • The absorption and emission of a single bonded molecule will have a distinct spectral signature. There may be a technology/instrument which is capable of probing the spectral signature of a sample containing a single molecule.
  • As mentioned above, the mass-field response of an atom and molecule may be measured by ionization, acceleration, and field deflection of particles at a different radius depending on its ionization state and mass. Such technology may accelerate a single molecule or atom, but normally a larger sample is vaporized and its mass measured by this method. Such a method separates bonded from the unbonded atoms by their radius of deflection.
  • And, we may infer the presence of an atom or molecule by the quintessential quantum phenomenon - the de Broglie wavelength of a high-velocity mass. While requiring multiple particles to form a pattern, we may accelerate ionized atoms/molecules, pass them through a double slit interferometer, and observe the unique spacing of light and dark fringes for each mass at a given velocity. The spacing created by the unknown sample thus reflects its mass, and therefore the presence or lack of bonding in the unknown sample.
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