How do scientists experimentally (this means not theoretically) determine that an atom is not bonded with any other atom? 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. 
 A: 
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?



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*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: 


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*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.



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*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. 

