THE ATOMIC THEORY
~Three fundamental laws of matter.~ Before we can gain any very definite
idea in regard to the structure of matter, and the way in which
different kinds of substances act chemically upon each other, it is
necessary to have clearly in view three fundamental laws of matter.
These laws have been established by experiment, and any conception which
may be formed concerning matter must therefore be in harmony with them.
The laws are as follows:
~Law of conservation of matter.~ This law has already been touched upon in
the introductory chapter, and needs no further discussion. It will be
recalled that it may be stated thus: _Matter can neither be created nor
destroyed, though it can be changed from one form into another._
~Law of definite composition.~ In the earlier days of chemistry there was
much discussion as to whether the composition of a given compound is
always precisely the same or whether it is subject to some variation.
Two Frenchmen, Berthollet and Proust, were the leaders in this
discussion, and a great deal of most useful experimenting was done to
decide the question. Their experiments, as well as all succeeding ones,
have shown that the composition of a pure chemical compound is always
exactly the same. Water obtained by melting pure ice, condensing steam,
burning hydrogen in oxygen, has always 11.18% hydrogen and 88.82% oxygen
in it. Red oxide of mercury, from whatever source it is obtained,
contains 92.6% mercury and 7.4% oxygen. This truth is known as _the law
of definite composition_, and may be stated thus: _The composition of a
chemical compound never varies._
~Law of multiple proportion.~ It has already been noted, however, that
hydrogen and oxygen combine in two different ratios to form water and
hydrogen dioxide respectively. It will be observed that this fact does
not contradict the law of definite composition, for entirely different
substances are formed. These compounds differ from each other in
composition, but the composition of each one is always constant. This
ability of two elements to unite in more than one ratio is very
frequently observed. Carbon and oxygen combine in two different ratios;
nitrogen and oxygen combine to form as many as five distinct compounds,
each with its own precise composition.
In the first decade of the last century John Dalton, an English
school-teacher and philosopher, endeavored to find some rule which holds
between the ratios in which two given substances combine. His studies
brought to light a very simple relation, which the following examples
will make clear. In water the hydrogen and oxygen are combined in the
ratio of 1 part by weight of hydrogen to 7.94 parts by weight of oxygen.
In hydrogen dioxide the 1 part by weight of hydrogen is combined with
15.88 parts by weight of oxygen. The ratio between the amounts of oxygen
which combine with the same amount of hydrogen to form water and
hydrogen dioxide respectively is therefore 7.94: 15.88, or 1: 2.
[Illustration: JOHN DALTON (English) (1766-1844)
Developed the atomic theory; made many studies on the properties and the
composition of gases. His book entitled "A New System of Chemical
Philosophy" had a large influence on the development of chemistry]
Similarly, the element iron combines with oxygen to form two oxides, one
of which is black and the other red. By analysis it has been shown that
the former contains 1 part by weight of iron combined with 0.286 parts
by weight of oxygen, while the latter contains 1 part by weight of iron
combined with 0.429 parts by weight of oxygen. Here again we find that
the amounts of oxygen which combine with the same fixed amount of iron
to form the two compounds are in the ratio of small whole numbers, viz.,
2:3.
Many other examples of this simple relation might be given, since it has
been found to hold true in all cases where more than one compound is,
formed from the same elements. Dalton's law of multiple proportion
states these facts as follows: _When any two elements,_ A _and_ B,
_combine to form more than one compound, the amounts of_ B _which unite
with any fixed amount of_ A _bear the ratio of small whole numbers to
each other_.
~Hypothesis necessary to explain the laws of matter.~ These three
generalizations are called _laws_, because they express in concise
language truths which are found by careful experiment to hold good in
all cases. They do not offer any explanation of the facts, but merely
state them. The human mind, however, does not rest content with the mere
bare facts, but seeks ever to learn the explanation of the facts. A
suggestion which is offered to explain such a set of facts is called an
_hypothesis_. The suggestion which Dalton offered to explain the three
laws of matter, called the _atomic hypothesis_, was prompted by his view
of the constitution of matter, and it involves three distinct
assumptions in regard to the nature of matter and chemical action.
Dalton could not prove these assumptions to be true, but he saw that if
they were true the laws of matter become very easy to understand.
~Dalton's atomic hypothesis.~ The three assumptions which Dalton made in
regard to the nature of matter, and which together constitute the atomic
hypothesis, are these:
1. All elements are made up of minute, independent particles which
Dalton designated as _atoms_.
2. All atoms of the same element have equal masses; those of different
elements have different masses; in any change to which an atom is
subjected its mass does not change.
3. When two or more elements unite to form a compound, the action
consists in the union of a definite small number of atoms of each
element to form a small particle of the compound. The smallest particles
of a given compound are therefore exactly alike in the number and kinds
of atoms which they contain, and larger masses of the substances are
simply aggregations of these least particles.
~Molecules and atoms.~ Dalton applied the name atom not only to the minute
particles of the elements but also to the least particles of compounds.
Later Avogadro, an Italian scientist, pointed out the fact that the two
are different, since the smallest particle of an element is a unit,
while that of a compound must have at least two units in it. He
suggested the name _molecule_ for the least particle of a compound which
can exist, retaining the name _atom_ for the smallest particle of an
element. In accordance with this distinction, we may define the atom and
the molecule as follows: _An atom is the smallest particle of an element
which can exist. A molecule is the smallest particle of a compound which
can exist._ It will be shown in a subsequent chapter that sometimes two
or more atoms of the same element unite with each other to form
molecules of the element. While the term atom, therefore, is applicable
only to elements, the term molecule is applicable both to elements and
compounds.
~The atomic hypothesis and the laws of matter.~ Supposing the atomic
hypothesis to be true, let us now see if it is in harmony with the laws
of matter.
1. _The atomic hypothesis and the law of conservation of matter._ It is
evident that if the atoms never change their masses in any change which
they undergo, the total quantity of matter can never change and the law
of conservation of matter must follow.
2. _The atomic hypothesis and the law of definite composition._
According to the third supposition, when iron combines with sulphur the
union is between definite numbers of the two kinds of atoms. In the
simplest case one atom of the one element combines with one atom of the
other. If the sulphur and the iron atoms never change their respective
masses when they unite to form a molecule of iron sulphide, all iron
sulphide molecules will have equal amounts of iron in them and also of
sulphur. Consequently any mass made up of iron sulphide molecules will
have the same fraction of iron by weight as do the individual iron
sulphide molecules. Iron sulphide, from whatever source, will have the
same composition, which is in accordance with the law of definite
composition.
3. _The atomic hypothesis and the law of multiple proportion._ But this
simplest case may not always be the only one. Under other conditions one
atom of iron might combine with two of sulphur to form a molecule of a
second compound. In such a case the one atom of iron would be in
combination with twice the mass of sulphur that is in the first
compound, since the sulphur atoms all have equal masses. What is true
for one molecule will be true for any number of them; consequently when
such quantities of these two compounds are selected as are found to
contain the same amount of iron, the one will contain twice as much
sulphur as the other.
The combination between the atoms may of course take place in other
simple ratios. For example, two atoms of one element might combine with
three or with five of the other. In all such cases it is clear that the
law of multiple proportion must hold true. For on selecting such numbers
of the two kinds of molecules as have the same number of the one kind of
atoms, the numbers of the other kind of atoms will stand in some simple
ratio to each other, and their weights will therefore stand in the same
simple ratio.
~Testing the hypothesis.~ Efforts have been made to find compounds which
do not conform to these laws, but all such attempts have resulted in
failure. If such compounds should be found, the laws would be no longer
true, and the hypothesis of Dalton would cease to possess value. When an
hypothesis has been tested in every way in which experiment can test it,
and is still found to be in harmony with the facts in the case, it is
termed a _theory_. We now speak of the atomic theory rather than of the
atomic hypothesis.
~Value of a theory.~ The value of a theory is twofold. It aids in the
clear understanding of the laws of nature because it gives an
intelligent idea as to why these laws should be in operation.
A theory also leads to discoveries. It usually happens that in testing a
theory much valuable work is done, and many new facts are discovered.
Almost any theory in explaining given laws will involve a number of
consequences apart from the laws it seeks to explain. Experiment will
soon show whether these facts are as the theory predicts they will be.
Thus Dalton's atomic theory predicted many properties of gases which
experiment has since verified.
~Atomic weights.~ It would be of great advantage in the study of chemistry
if we could determine the weights of the different kinds of atoms. It is
evident that this cannot be done directly. They are so small that they
cannot be seen even with a most powerful microscope. It is calculated
that it would take 200,000,000 hydrogen atoms placed side by side to
make a row one centimeter long. No balance can weigh such minute
objects. It is possible, however, to determine their relative
weights,--that is, how much heavier one is than another. _These relative
weights of the atoms are spoken of as the atomic weights of the
elements._
If elements were able to combine in only one way,--one atom of one with
one atom of another,--the problem of determining the atomic weights
would be very simple. We should merely have to take some one convenient
element as a standard, and find by experiment how much of each other
element would combine with a fixed weight of it. The ratios thus found
would be the same ratios as those between the atoms of the elements, and
thus we should have their relative atomic weights. The law of multiple
proportion calls attention to the fact that the atoms combine in other
ratios than 1: 1, and there is no direct way of telling which one, if
any, of the several compounds in a given case is the one consisting of a
single atom of each element.
If some way were to be found of telling how much heavier the entire
molecule of a compound is than the atom chosen as a standard,--that is,
of determining the molecular weights of compounds,--the problem could be
solved, though its solution would not be an entirely simple matter.
There are ways of determining the molecular weights of compounds, and
there are other experiments which throw light directly upon the relative
weights of the atoms. These methods cannot be described until the facts
upon which they rest have been studied. It will be sufficient for the
present to assume that these methods are trustworthy.
~Standard for atomic weights.~ Since the atomic weights are merely
relative to some one element chosen as a standard, it is evident that
any one of the elements may serve as this standard and that any
convenient value may be assigned to its atom. At one time oxygen was
taken as this standard, with the value 100, and the atomic weights of
the other elements were expressed in terms of this standard. It would
seem more rational to take the element of smallest atomic weight as the
standard and give it unit value; accordingly hydrogen was taken as the
standard with an atomic weight of 1. Very recently, however, this unit
has been replaced by oxygen, with an atomic weight of 16.
~Why oxygen is chosen as the standard for atomic weights.~ In the
determination of the atomic weight of an element it is necessary to find
the weight of the element which combines with a definite weight of
another element, preferably the element chosen as the standard. Since
oxygen combines with the elements far more readily than does hydrogen to
form definite compounds, it is far better adapted for the standard
element, and has accordingly replaced hydrogen as the standard. Any
definite value might be given to the weight of the oxygen atom. In
assigning a value to it, however, it is convenient to choose a whole
number, and as small a number as possible without making the atomic
weight of any other element less than unity. For these reasons the
number 16 has been chosen as the atomic weight of oxygen. This makes
the atomic weight of hydrogen equal to 1.008, so that there is but
little difference between taking oxygen as 16 and hydrogen as 1 for the
unit.
The atomic weights of the elements are given in the Appendix.
EXERCISES
1. Two compounds were found to have the following compositions: (a)
oxygen = 69.53%, nitrogen = 30.47%; (b) oxygen = 53.27%, nitrogen =
46.73%. Show that the law of multiple proportion holds in this case.
2. Two compounds were found to have the following compositions: (a)
oxygen = 43.64%, phosphorus = 56.36%; (b) oxygen = 56.35%, phosphorus
= 43.65%. Show that the law of multiple proportion holds in this case.
3. Why did Dalton assume that all the atoms of a given element have the
same weight?