CHEMISTRY CHAPTER ZERO

Evidence for Atoms

Why should I accept atoms as real?

Note that the language of this question guides the discussion away from the “Do you believe in atoms?” The word “believe” has many facets – religion, affirmation, acceptance, opinion, etc – associated with it, and it is better to guide the question toward “What experimental data will guide me?”

You will need a good definition of an atom. Remember that the word atom comes from the Greek "a" [not] + "tomos" [cut], i.e., the last particle, the particle that cannot be further subdivided.

The experimental data presented here addresses two sets of questions:

-- Are there really “last particles”? If we could see the details, would we find that the world is really granular, i.e, more like sand than like jello?

-- If the world is made of “last particles”, are all the particles the same? In particular, given our questions about ripping matter apart and weighing it, do all the particles have the same weight?

The experimental data is presented here. The question of “How do I ‘see atoms without eye seeing’?” is put aside for now.

The standard argument follows the historical development of chemistry. Dalton made careful measurements of the weights of compounds, Proust postulated that atoms combine in the ratios of small integers, Avogadro’s Principle was used to resolve the question of the structure of water (H + 2O or H + O), etc. While the argument is correct, I doubt that it convinces many ninth graders – particularly pre-algebra students – and I have concentrated on data that is likely to be compelling without algebra.

Atomic Force Microscopy (AFM)

The point to drive home here is that materials are made up of "lumps", i.e., atoms.

In AFM a lever assembly which has a very sharp point is scanned across a sample. The point of the assembly will track the height of the surface. By reading out the deflection of the lever, a plot of the surface height can be prepared.

The AFM “feels’s the surface, just as a blind person might accurately describe an object just by feeling it with the fingers. Just as a student might feel the marbles in a box..

The following web sites (among many) present the AFM technique:

http://www.mee-inc.com/afm.html

Consider the AFM data shown in the following images (by convention brighter parts of the image represent higher areas of the surface)

AFM image of a surface of gold atoms on mica

AFM image of a surface of a salt (NaCl) crystal

Or write IBM with atoms!

Hopefully students can be guided to interpret these images as showing high and low points, and in the end, as showing discrete particles making up the surface. If they do so, then they have accepted that, these materials are made up of discrete lumps, i.e, “atoms”.

 

Mass Spectrometry (MS)

The point to drive home is that the lumps [atoms] have different weights.

In MS, particles are vaporized, ionized, and accelerated with an electric field. This results in all particles, regardless of their weight, having the same final kinetic energy. The ions are then passed through a “mass filter” in which the ions acquire different trajectories depending upon their different masses. If one measures peaks in the signal as the trajectories are varied, one should conclude that there are particles (ions) with differing weights.

The following web sites present the MS technique:

http://www.chem.arizona.edu/massspec/example_html/examples.html

The figure below shows the mass spectrum of ethanol, the molecule ("a group of atoms that move together"), the molecule that is featured in the bottom corners.

 

Aha! A molecule is made up of atoms. And when we break up the molecule (partially) in the process of obtaining the mass spectrum, we see the masses of the parts. Hopefully, at this point, students will accept that atoms of different elements have different weights.

Note: A student may say “But I don’t understand how this instrument works. I don’t have to accept the data as proof.” The key issue here is “How do you know and accept things that are beyond your current five senses?” I think the best answer is along the lines of “The data is there and is real. You could apply yourself and learn how these instruments work. I do not ask you to accept atoms on the authority of the textbook, but I do ask you to examine data, and if necessary, to learn to examine data.”

Or...you might make use of the AFM and MS hands-on materials described in this web site. The students may understand how the real AFM and MS work once they have used the hands-on versions.