ie-Chemistry P2
Sizes of Atoms and Molecules


John DaltonThe previous investigation suggested several ways to determine the thickness of a thin layer, either as an oil slick or a bubble.  Benjamin Franklin (b1706, d1790) realized that such an investigation could be used to provide information about the size of atoms.  In Franklin's time, atoms were discussed.  Many people thought that the original Greek idea of a smallest particle of matter remained plausible.  But there was no firm evidence that atoms really existed.  Soon Chemists, such as the Englishman John Dalton (b1766, d1844, portrait at right→), would be making significant progress based on the assumption of the existence of atoms.  But for over a century more, some very prominent scientists continued to note that while the atomic hypothesis provided a productive basis for research, there was still a lack of any real evidence for the existence of atoms.

Not until work such as Albert Einstein's (b1879, d1955) 1905 paper mathematically relating observed Brownian motion to the kinetic energy of moving air molecules did the consensus of scientists swing to hold that atoms are real.

Over millennia, human beings living in different situations developed different cultures and different languages.  Such differences contributed to animosities between people but have also provided a rich pool of useful ideas and vocabularies.  Where English speaking scientists talked of atoms, others such as the Italian Amadeo Avogadro (1776 to 1856) spoke of molecules.  Eventually it became clear that a distinction was needed between the single, lone, smallest pieces of matter, and the very common, small, uniform aggregates of these smallest pieces.  The former became known as atoms and the latter molecules.  For example, the smallest particle of air is a Nitrogen atom.  But Nitrogen (symbol N) doesn't exist in air as lone atoms, but rather bonds together in pairs which we call Nitrogen molecules (symbol N2).


This distinction becomes important when we consider the information from measuring the thickness of a soap bubble or the depth of an oil slick.  Both soap and oil are composed of molecules.  So when we measure the thickness of the thinnest we can make such a layer, we are most likely measuring the size of the molecules in the layer.

If we know a bit about the size and arrangement of molecules in our layer, we can estimate the size of an atom!  We can simply take the results of the previous investigation and apply our understanding about the molecules to calculate a reasonable measure of an atom's size.  (Are you noticing here that sometimes advances in our understanding come from doing experiments, making observations and measurements, while at other times they come from connecting ideas and making calculations?)

Soap and oil are rather broad descriptions of molecules.  There are many kinds of soap molecules and many types of oily substances.  So the better you understand the particular soap or oil you use to form a thin layer, the more accurate will be your estimate of the size of an atom.  Since oils are typically the simpler, we will consider them first.  There are basically two types of oils, simple hydrocarbons (composed of just Hydrogen and Carbon atoms) and lipids (with a few additional Oxygen atoms).  Lipids are produced by living organisms which use them for a wide variety of functions.  We purify and sell them commercially as eatable oils such as corn oil, peanut oil, cod liver oil and similar.  Hydrocarbons are generally found in petroleum which presumably degenerated from decayed and pressure cooked dead organisms.  Hydrocarbons are purified and sold as mineral oil, various viscosity lubricating oils, petrol, and waxes.  Soaps are generally made by substituting some additional atoms to lipids.

Hydrocarbons are generally long chains of Carbon atoms surrounded by a covering of Hydrogen atoms.  They primarily differ in the length of the Carbon chain.  We know the 3 Carbon chain as propane, the longer in gasoline such as octane with 8 Carbons, kerosene a range from 6 to 16 Carbon atoms, lubricating oils often having 18 to 30 Carbons and waxes and tars contain even longer lengths.  The longer chain lengths are more viscous, providing us an easy clue to the approximate length of the hydrocarbon.  The Carbon and Hydrogen atoms share the valence electron nearly equally resulting in molecules with almost no uneven distribution of electric charge.  The resulting non-polar molecules thus have little attraction to molecules such as water where the Oxygen hoards valence electrons making an electrically lopsided, polar molecule.  So we might imagine that in the thinnest layer of hydrocarbons floating on a pond of water, the hydrocarbon chains might exist as moderately tangled balls or at best laying down on their sides making the layer 3 to 6 atoms thick.

Lipids have more diversity than hydrocarbons.  But most common lipids, the triglycerides, have hydrocarbon chains 12 to 18 carbons long.  On one end of each such chain is an organic function group (which gets its name because it changes the chemical behavior of the molecule) containing two Oxygen atoms.  Three of these chains are usually linked together by a three Carbon, three Oxygen glycerol (thus the combined name of triglyceride).  As noted above, the Oxygen doesn't share valence electrons equally with the Carbon.  The result is that the glycerol (its original name was glycerine) and function group end of the large molecule have a strong attraction for water, while the non-polar hydrocarbon chains do not.  As a result we can expect lipids to align with their Oxygen's implanted against the surface of the water, leaving the hydrocarbon tails crowded like a forest of trees standing upright.  Thus a lipid layer may be at most 20 atoms deep, but presuming some sag in the hydrocarbon chains, perhaps only 8 to 12 atoms deep.

Lipids can be disassembled into triglyceride and separate hydrocarbon chains, each with its functional group, called fatty acids.  These may be available commercially.  Fatty acids make good layers which can be measured.  But glycerol can't be used to form a layer on water because it is water soluble although it can be used to make more lasting bubbles.

Soaps are typically made by reacting lipids with a base such as lye, NaOH.  (Caution, lye is a very corrosive substance since skin and other cell membranes are composed of lipids; wear goggles and protective clothing if you experiment with lye.)  Most of the lye is consumed in the reaction (although if any remains, it will cause stinging if that soap gets in your eye).  The lipid is disassembled producing glycerol and fatty acids with the Sodium, Na, or similar attached to the functional group.  This Sodium salt of the fatty acid is what we know as soap!  As described above, the Sodium and Oxygen on the end of the fatty acid are each attracted to water.  But in this case the ionic attraction is so strong that the entire soap is pulled into the water, where it may form little droplets with hundreds of other soap molecules with the hydrocarbon tails entangled in the core of the droplet, and all the functional groups aligned outward forming the surface of the bubble.  Such soap droplets are often so large that they diffract light, making the water containing the soap a bit cloudy.

A very concentrated solution of soap and water (plus perhaps glycerol) can form thin films such as in bubbles or on wire frames.  Presumably since air is non-polar, each soap film is composed of two layers of soap with their hydrocarbon tails outward toward the air and their polar functional groups towards a thin intermediary layer of water (plus perhaps glycerol). So the soap film may be twice as think as a lipid layer, perhaps 20 atoms thick.


  1. Determine what materials was used to create the film layer or bubble.

  2. Research as much as you wish to determine the composition of the film and the most likely number of atoms composing the thickness of the layer.

  3. Divide the thickness of the layer (calculated in the previous investigation) by the number of atoms of thickness.

  4. You may want to make layers from a number of different substances and compare your results.

  5. Record your procedure and observations in your journal along with any analysis and conclusions which seemed appropriate.

  6. Communicating technical information such as observations and findings is a skill used by scientists but useful for most others.  If you need course credit, use your observations in your journal to construct a formal report.


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content created 11 July 2009
by D Trapp
Mac made