Early Evidence of Atomic Structure

 "Understanding means seeing that the same thing said different ways is the same thing." --- Ludwig Wittgenstein


Democritus was a fifth century BCE mathematician who postulated the existence if atoms as the smallest pieces of matter. Check these web sites to determine how he came to this conclusion and how atomic theory ebbed and flowed down through the ages.

 mod-01  http://www-groups.dcs.st-and.ac.uk/~history/Mathematicians/Democritus.html

To see where all this is going, check out the atomic physics time line at

2. John Dalton

Dalton was a British chemist whose career spanned the 18th & 19th centuries. His initial goal in studying chemistry was to debunk the idea of alchemy - the alleged process to turn base metals into gold. Along the way he established some general rules for the behavior of atoms that guided chemists for virtually the entire 19th century.

Dalton's Rules
Matter consists of indivisible atoms.
Dalton established himself as an atomist and saw no evidence to suggest that atoms could be divided into simpler parts.

2. Each element consists of a characteristic kind of identical atom.
The suggestion here is that all atoms of an element are alike but the atoms of each element are different. In retrospect, there is a hint here of some kind of atomic structure. There must be some physical feature that makes each atom different.

3 Atoms are unchangeable.
It is here that he puts the nail in the alchemy coffin. He can find no evidence that base metals are but unripened gold. Rather, every gold atom, indeed every kind of atom, has been on the planet forever.

4. Atoms combine to form compounds in definite ratios of small, whole numbers.
Again the order thing, a definite recipe for each compound. This does not necessarily rule out making more than one compound from two elements (see carbon & hydrogen)

5 In chemical reactions, atoms are neither created nor destroyed; they are rearranged.
This is a statement of the law of conservation of mass, a rule not immediately obvious during combustion, for instance. Throughout the nineteenth century, chemists confirmed again and again that the mass of the products of a chemical change event is always the same as the mass of the reactants.

The learned reader may be wondering by now why these five rules are presented here. After all, rules 1, 2, & 3 are not valid today. And rule 5 is only true if we underscore chemical reactions rather than nuclear reactions. Later in this unit we will be reading of the degree of astonishment that scientists had as new discoveries caused new theories to be developed. One can only appreciate a new theory by holding it up against the well-established standard.

3. The Periodic Table

In the modern high school classroom, one can determine what is taught there by searching for tell-tale artifacts; a biology class might render collection of microscopes, an astronomy class might be given away by a telescope. In chemistry, the space is given away by the Periodic Table. The usual multi-colored wall chart is more than just a pretty wall-hanging; it represents a significant piece of history in the development of atomic theory.
By 1871 some 63 elements were known to exist. A fundamental point of interest among chemists concerned whether or not there existed a connection among them. In 1829, Johann Dobereiner suggested that some elements, when listed by increasing atomic mass, could be grouped in threes (he called them triads).

While others were able to discern parts of the pattern, Dmitri Mendeleev used an uncommonly thorough knowledge of the properties of each element to lay the foundation for the modern periodic table. His passion for the elements bordered on obsession. He carried with him a set of cards, one for each element, whereupon he wrote the complete set of known physical properties. In a rudimentary game of solitaire, he would lay the cards in a row on a table by increasing atomic mass. He started with lithium, beryllium, boron, carbon, nitrogen, oxygen and fluorine. The next element known to him was sodium. Rather than continue the row, he started a new row and put sodium in the same column with lithium (which has properties similar to sodium). Next in order come magnesium aluminum, silicon, phosphorus, sulfur, and chlorine. Each of these elements has chemical and physical properties very much like the element immediately above it. (Mendeleev called these groupings "families". ) The next element is potassium and it has a family resemblance to lithium and sodium; potassium starts a new row. Thus, the same properties show up in the list at regular intervals, "periodically" one might call it. He published this speculation in as paper, On the Relation of the Properties to the Atomic Weights of the Elements. His table looked like something this.

1A  2A  1B  2B  3B  4B  5B  6B  7B  8B  9B  1B  3A  4A  5A  6A  7A  8A


 Be                      B  C  N  O  F  

 Mg                      Al  Si  P  S  Cl  

 Ca    Ti  V  Cr  Mn  Fe  Co  Ni  Cu  Zn      As  Se  Br  

 Sr  Y  Zr  Nb  Mo    Ru Rh   Pd  Ag  Cd  In  Sn  Sb  Te  I  

 Ba  La    Ta  W    Os  Ir  Pt  Au  Hg  Tl  Pb  Bi      
       Ce              Tb              
       Th    U                        

These are the elements that were known to Mendeleev by 1871, listed here for the most part by increasing atomic mass. But "for the most part" does not mean "always". For you see, there are reversals,a) where the mass of Cobalt is slightly more that the mass of Nickel; b) where the mass of Tellurium is greater than the mass of Iodine; In these two cases, he concluded that periodicity of chemical and physical properties took precedence over atomic mass.
Organizing the 63 then-known elements was a sufficient feat in itself; Mendeleev brought order out of chaos. And then he did more. As he was building the table ,element by element, he realized that he did not have all of the data for some cells the ones in yellow. Clinging tightly to the belief that the elements in a column should share similar properties, he left some cells blank, putting the next-heavier element in a column with its family. Then by using the properties of adjacent elements as a guide, he was able to
predict what would be the properties of the unknown element once it was discovered.
Perhaps an analogy would drive home the point. Let's consider a useful analogy. The accepted plan of attack when assembling a jig saw puzzle is to assemble the edges first and then work on the patterns of the picture being assembled. What is the likelihood that with only the border assembled in a 1000 piece puzzle, should I place my finger on an interior space, you can proceed to select precisely the required piece. Your chances of success are about 1 in 1000. Now consider the task when we know the shape and color of the pieces left and right, top and bottom.One's chances of success are greatly improved.

Mendeleev did this for four vacancies. In 1872, he predicted with great exactness the properties of the occupant of periodic table cell #32, which he called eka-silica. He said that the new element should have a density of 5.5 g/cm^3, an oxide whose density was 4.7 g/cm^3, and a compound with chlorine that had a density of 1.9 g/cm^3 A element was found to have those exact properties in 1886 and was named germanium.

His scheme was so good that he was able to predict the properties of elements that had yet to be discovered. One could conclude that the order brought to the elements had far-reaching consequences. Mendeleev's work raises the new question "What is it about the structure of the elements in a column that gives the elements in that column very similar chemical and physical properties?"

There is a structure waiting to be uncovered.

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Last edited 07/05/08