Download Dalton Model Reading

Dalton Model
Excerpted from Wikipedia on July 9, 2010
Atomic weights
Dalton proceeded to print his first published table of relative atomic weights. Six
elements appear in this table, namely hydrogen, oxygen, nitrogen, carbon, sulfur, and
phosphorus, with the atom of hydrogen conventionally assumed to weigh 1. Dalton
provided no indication in this first paper how he had arrived at these numbers. However,
in his laboratory notebook under the date 6 September 1803 there appears a list in which
he sets out the relative weights of the atoms of a number of elements, derived from
analysis of water, ammonia, carbon dioxide, etc. by chemists of the time.
In 1803 Dalton orally presented his first list of relative atomic weights for a number of
substances. This paper was published in 1805, but he did not discuss there exactly how he
obtained these figures. The method was first revealed in 1807 by his acquaintance
Thomas Thomson, in the third edition of Thomson's textbook, A System of Chemistry.
Finally, Dalton published a full account in his own textbook, A New System of Chemical
Philosophy, 1808 and 1810.
Dalton estimated the atomic weights according to the mass ratios in which they
combined, with hydrogen being the basic unit. However, Dalton did not conceive that
with some elements atoms exist in molecules — e.g. pure oxygen exists as O2. He also
mistakenly believed that the simplest compound between any two elements is always one
atom of each (so he thought water was HO, not H2O). This, in addition to the crudity of
his equipment, resulted in his table being highly flawed. For instance, he believed oxygen
atoms were 5.5 times heavier than hydrogen atoms, because in water he measured
5.5 grams of oxygen for every 1 gram of hydrogen and believed the formula for water
was HO (an oxygen atom is actually 16 times heavier than a hydrogen atom).
Atomic Theory
Dalton proposed that each chemical element is composed of atoms of a single, unique
type, and though they cannot be altered or destroyed by chemical means, they can
combine to form more complex structures (chemical compounds). This marked the first
truly scientific theory of the atom, since Dalton reached his conclusions by
experimentation and examination of the results in an empirical fashion.
The flaw in Dalton's theory was corrected in 1811 by Amedeo Avogadro. Avogadro had
proposed that equal volumes of any two gases, at equal temperature and pressure, contain
equal numbers of molecules (in other words, the mass of a gas's particles does not affect
its volume). Avogadro's law allowed him to deduce the diatomic nature of numerous
gases by studying the volumes at which they reacted. For instance: since two liters of
hydrogen will react with just one liter of oxygen to produce two liters of water vapor (at
constant pressure and temperature), it meant a single oxygen molecule splits in two in
order to form two particles of water. Thus, Avogadro was able to offer more accurate
estimates of the atomic mass of oxygen and various other elements, and firmly
established the distinction between molecules and atoms.
Five main points of Dalton's atomic theory
1. The atoms of a given element are different from those of any other element; the
atoms of different elements can be distinguished from one another by their
respective relative atomic weights.
2. All atoms of a given element are identical.
3. Atoms of one element can combine with atoms of other elements to form
chemical compounds; a given compound always has the same relative numbers of
types of atoms.
4. Atoms cannot be created, divided into smaller particles, nor destroyed in the
chemical process; a chemical reaction simply changes the way atoms are grouped
together.
5. Elements are made of tiny particles called atoms.
Dalton proposed an additional "rule of greatest simplicity" that created controversy, since
it could not be independently confirmed.
When atoms combine in only one ratio, "..it must be presumed to be a binary one,
unless some cause appear to the contrary".
This was merely an assumption, derived from faith in the simplicity of nature. No
evidence was then available to scientists to deduce how many atoms of each element
combine to form compound molecules. But this or some other such rule was absolutely
necessary to any incipient theory, since one needed an assumed molecular formula in
order to calculate relative atomic weights. In any case, Dalton's "rule of greatest
simplicity" caused him to assume that the formula for water was OH and ammonia was
NH, quite different from our modern understanding.
Despite the uncertainty at the heart of Dalton's atomic theory, the principles of the theory
survived. To be sure, the conviction that atoms cannot be subdivided, created, or
destroyed into smaller particles when they are combined, separated, or rearranged in
chemical reactions is inconsistent with the existence of nuclear fusion and nuclear fission,
but such processes are nuclear reactions and not chemical reactions. In addition, the idea
that all atoms of a given element are identical in their physical and chemical properties is
not precisely true, as we now know that different isotopes of an element have slightly
varying weights. However, Dalton had created a theory of immense power and
importance.
Law of Definite Proportions
Near the end of the 18th century, two laws about chemical reactions emerged without
referring to the notion of an atomic theory. The first was the law of conservation of mass,
formulated by Antoine Lavoisier in 1789, which states that the total mass in a chemical
reaction remains constant (that is, the reactants have the same mass as the products). The
second was the law of definite proportions. First proven by the French chemist Joseph
Louis Proust in 1799, this law states that if a compound is broken down into its
constituent elements, then the masses of the constituents will always have the same
proportions, regardless of the quantity or source of the original substance.
John Dalton studied and expanded upon this previous work and developed the law of
multiple proportions: if two elements can together form more than one compound, then
the ratios of the masses of the second element which combine with a fixed mass of the
first element will be ratios of small integers. For instance, Proust had studied tin oxides
and found that their masses were either 88.1% tin and 11.9% oxygen or 78.7% tin and
21.3% oxygen (these were tin(II) oxide and tin dioxide respectively). Dalton noted from
these percentages that 100g of tin will combine either with 13.5g or 27g of oxygen; 13.5
and 27 form a ratio of 1:2. Dalton found an atomic theory of matter could elegantly
explain this common pattern in chemistry - in the case of Proust's tin oxides, one tin atom
will combine with either one or two oxygen atoms.
He hypothesized the structure of compounds can be represented in whole number ratios.
So, one atom of element X combining with one atom of element Y is a binary compound.
Furthermore, one atom of element X combining with two elements of Y or vice versa, is
a ternary compound. Many of the first compounds listed in the New System of Chemical
Philosophy correspond to modern views, although many others do not.
Dalton also believed atomic theory could explain why water absorbed different gases in
different proportions: for example, he found that water absorbed carbon dioxide far better
than it absorbed nitrogen. Dalton hypothesized this was due to the differences in mass
and complexity of the gases' respective particles. Indeed, carbon dioxide molecules (CO2)
are heavier and larger than nitrogen molecules (N2).