Dalton’s theory of atoms
About two thousand five hundred years ago, two philosophers from
Ancient Greece – Democritus and Leucippus by name – expressed what
seemed at that time to be the rather unlikely opinion that matter was, in the final
analysis, ultimately composed of some indivisible particles which they named a +
temnow (ατεμνω, and hence άτομος), which is Greek for indivisible. Now, these
atomists were more or less the last of the nature philosophers (Ancient Greek thinkers
who were concerned with the question of discovering the ultimate nature of matter).
The Atomists, because of the remarkable ‘correctness’ of their idea from the modern
perspective, have earned a firm and well deserved place in the early history of
philosophy. Incidentally, there was also an Indian atomic theory, which was
expounded somewhat earlier than the Greek one, whether there was some crossfertilisation between these ideas, particularly given the proximity of the two
civilisations, is unclear. The Indian atomism began with an analysis of how linguistic
structures acquire their meanings, and was propounded by Yaska. From there these
ideas seem to have transmogrified into notions about the fundamental structure of
reality, but this is beyond the scope of this post.
Source: (Bust of Leucippus - left) http://webspace.ship.edu/cgboer/greeks.html (Bust o
Democritus - right) http://faculty.washington.edu/smcohen/320/Democritus.jpg
The credit that is due to these atomists however is not at all for having
discovered modern atomic theory. Contrary to some misguided, “new-age” views, the
atomists, in fact, did nothing of the sort. What credit is due to them goes for three
things:
 For producing an interesting challenge to a rather alarming (at least to the other
Ancient Greek philosophers) set of arguments propounded by Parmenides and
his pupil Zeno, who were predecessors to the atomists, and who claimed that
in reality, things cannot move (arguments which I shall discuss in some future
column);
 For contributing to the advancement of epistemology (the philosophy of
knowledge);

And for adducing some remarkably modern notions of how the world works,
by reasoning in a logical manner from a set of assumptions that, in a sense,
simply happened to be right.
Another remarkable aspect of the ideas propounded by the Atomists was how
close they were in many ways to modern atomic theory. According to them:
1. atoms are indivisible (in fact, we now know that this is not true, but we also
know that if we divide an atom of say uranium, the remnants are no longer
uranium, but some other element say caesium and rubidium for example);
2. there are many different kinds of atoms, each with a distinct size and shape
(again, in a way this parallels our modern understanding of atoms);
3. they are in constant motion (ditto);
4. they are invisible, being far too small to be detected by human senses (ditto);
5. they can combine and un-combine in a variety of ways, and do so by ‘hooking’
together, or by ‘unhooking’. When they hook together, they make the
macroscopic things which we see: growth is the result of atoms hooking
together, and decay is the reverse process: atoms unhooking (ditto);
6. there are only atoms and void (kind of ditto, except that there are also the 4
fundamental forces of nature, and depending on your affiliation, strings,
membranes, and a bizarre zoo of other entities out there);
The atomists’ ideas, needless to say, both when they were first propounded and
even now, were / are so contrary to most people’s everyday experience, that
throughout most of recorded history their ideas did not find wide acceptance. After all,
there are very few macroscopic experiences that clearly and unequivocally indicate the
discrete, particulate nature of matter, unless you know how to interpret them.
Anyway, even though unconvincing, the atomists theory, as well as their
explanations of many physical phenomena were so elegant that scholars continued to
preserve their ideas, and to transmit them to future generations. Eventually, in the
early nineteenth century, John Dalton, (1766-1844) a colour-blind English scientist
and school teacher whose only recreation appears to have been lawn bowling on
Thursday afternoons, hit on the same idea, viz., that matter must be discrete in nature.
In honour of Leucippus and Democritus, from whom he derived his inspired solution
to the question what are things made of (and no small thanks to an education grounded
in the classics), Dalton named the fundamental constituents of matter atoms.
Source: http://reichchemistry.wikispaces.com/A.+Grimner+and+J.+Condlin+Time+Line+Project
At this point, you may be wondering what, if anything, distinguished Dalton’s
conjecture of the atomic nature of matter, from the seemingly identical conclusion of
the atomists’. Well, the difference, and it’s an enormous one, is that Dalton backed up
his conjectures with some experimental evidence, and with some arguments which
were premised on empirical observations. This speaks to the heart of science. The
theories in science are backed up by empirical evidence. This is a far cry from claims
which are made on the basis of metaphysical or other appeals. Thus, for example,
there is good, empirical reason to accept Darwinian evolution, but no scientific
reasons for accepting biblical an explanation of how life came to be.
Not that Dalton was entirely right or anything – in fact he was very wrong on
one important point, like the Greek Atomists, he too believed that atoms were
indivisible, and had no internal structure, when in fact, we now know to the contrary
that they do, and that they in their turn are composed of even more fundamental
particles. However, Dalton did lay out the essential foundations of modern atomic
theory:
Firstly, Dalton argued that each element was composed of tiny pieces of matter
which he called atoms, and which, for a given element, were all identical in every
way, and which could not be further subdivided without destroying the element at
hand. An iron atom in other words is the smallest piece of iron that you can have.
Indeed, this is the very notion of an element: a chemically elementary particle, a
material which cannot be further decomposed or transformed into another material
using chemical means. In fact, the concept of elements too (stoicheia), like that of
atoms, comes from the ancient Greeks.
Secondly, according to Dalton, the atoms of each element were different from
those of any other element, and it was various aspects of these differences that
accounted for the differing properties of the elements. Hence, gold is a dense yellow
relatively non-reactive metal, while chlorine is a yellowish-green highly reactive gas
because the respective atoms of gold and chlorine have different properties.
Thirdly, he suggested that all compounds were composed of some combination
of atoms of more than one element. The ultimate unit of table salt for example is the
sodium chloride molecule, which is always composed of one sodium atom chemically
joined to one chlorine atom.
Fourthly, he maintained that just as the elements derived their properties from
the characteristics of their constituent atoms, so also did compounds get their
properties from specific combinations of different elemental atoms (methane (CH4)
and butane (C4H10) for example are both compounds composed of the elements
carbon and hydrogen, but methane and butane have different properties because of
their different ways in which carbon and hydrogen have been combined in these two
compounds).
Fifthly, he showed that the elements in a compound are always combined in
simple ratios (1:1, 2:5, 34:56, etc.). This in fact was the clincher for Dalton’s theory. If
compounds are always composed of simple ratios of elements, then it necessarily
follows that the ultimate constituents of matter must be discrete, as otherwise some
fraction of an atom could add itself to some other fraction of some other atom. What
Dalton found however was that the constituents of the many specific compounds
which he investigated were always present in fixed ratios. Thus, he reasoned, matter
must ultimately be composed of minute indivisible particles.
Finally, Dalton contended that atoms cannot be created or destroyed, and that
by extension, all chemical changes were merely separations, combinations or
rearrangements of atoms.
These were the basic laws of chemical behaviour that Dalton came up with,
based on his atomic hypothesis. They revolutionized people’s understanding of the
chemical behaviours of substances. Even though people knew about chemical
processes, Dalton’s work afforded them a fresh and convincing theoretical
understanding of the phenomena which they had been familiar with, in some cases (as
with the manufacture of glass, iron, or alcohol say) for millennia. Dalton’s thinking
also eventually spurred other scientists to do further thinking about atoms, and the
ultimate nature of matter, and in turn to come up with the stream of discoveries that
together constitutes atomic physics, but that is a story for another day.