Dalton's Model of Atom

2. …Today we will discuss
about
“Dalton’s Model of
Atom”

3. It was in the early 1800s that John Dalton,
an observer of weather and discoverer of color
blindness among other things, came up with his
atomic theory. Let's set the stage for Dalton's
work. Less than twenty years earlier, in the
1780's, Lavoisier ushered in a new chemical era by
making careful quantitative measurements which
allowed the compositions of compounds to be
determined with accuracy. By 1799 enough data
had been accumulated for Proust to establish the
Law of Constant Composition ( also called the Law
of Definite Proportions).

4. In 1803 Dalton noted that oxygen
and carbon combined to make two
compounds. Of course, each had its own
particular weight ratio of oxygen to
carbon (1.33:1 and 2.66:1), but also, for
the same amount of carbon, one had
exactly twice as much oxygen as the
other. This led him to propose the Law of
Simple Multiple Proportions, which was
later verified by the Swedish chemist
Berzelius. In an attempt to explain how
and why elements would combine with one
another in fixed ratios and sometimes
also in multiples of those ratios, Dalton
formulated his atomic theory.

5. The idea of atoms had been proposed
much earlier. The ancient Greek philosophers
had talked about atoms, but Dalton's theory
was different in that it had the weight of
careful chemical measurements behind it. It
wasn't just a philosophical statement that
there are atoms because there must be atoms.
His atomic theory, stated that elements
consisted of tiny particles called atoms. He
said that the reason an element is pure is
because all atoms of an element were identical
and that in particular they had the same mass.
He also said that the reason elements differed
from one another was that atoms of each
element were different from one another; in
particular, they had different masses.

6. He also said that compounds
consisted of atoms of different elements
combined together. Compounds are pure
substances (remember they cannot be
separated into elements by phase changes)
because the atoms of different elements
are bonded to one another somehow,
perhaps by hooks, and are not easily
separated from one another. Compounds
have constant composition because they
contain a fixed ratio of atoms and each atom
has its own characteristic weight, thus
fixing the weight ratio of one element to
the other. In addition he said that chemical
reactions involved the rearrangement of
combinations of those atoms.

7. So that, briefly, is Dalton's theory.
With modifications, it has stood up pretty
well to the criteria that we talked about
earlier. It did not convince everyone right
away however. Although a number of
chemists were quickly convinced of the
truth of the theory, it took about a half
century for the opposition to die down, or
perhaps I should say die off. Dalton's
model was that the atoms were tiny,
indivisible, indestructible particles and
that each one had a certain mass, size,
and chemical behavior that was
determined by what kind of element they
were.

8. However, only 3 or 4 pages in the third
chapter discussed the atomic theory he
proposed. In this theory, there are four basic
ideas...
1) chemical elements are made of atoms.
2) the atoms of an element are identical in their
mases
3) atoms of different elements have different
masses
4) atoms only combine in small, whole number
ratios such as 1:1, 1:2, 2:3 and so on.
5) atoms can be neither created nor destroyed

9. This is the Law of Multiple Proportions. Simply put, when
elements combine to form compounds they do so in whole number
ratios.
Carbon and Oxygen can form either CO ( a 1:1 ratio) or CO2
(a 1:2 ratio). It also states that if the mass of the first element is
constant the second elements masses would be in a whole number
ratio.
1.00g of Carbon will have combined with 1.33g in CO
1.00g of Carbon will have combined with 2.66g in CO2
The mass are in a 1:2 ratio
Simply put if combinations are in whole number ratios, there
must be a one. This one being an atom. Here are Dalton's
assumptions.
All matter is composed of atoms
Atoms cannot be made or destroyed
All atoms of the same element are identical
Different elements have different types of atoms
Chemical reactions occur when atoms are rearranged

10. Dalton’s atomic model is one of the
fundamentals of physics and chemistry. This theory
of atomic composition was hypothesized and
partially confirmed by the English chemist and
Physicist John Dalton. Dalton came with his Atomic
theory as a result of his research into gases. He
discovered that certain gases only could be
combined in certain proportions even if two
different compounds shared the same common
element or group of elements. Through deductive
reasoning and experimentation, he made an
interesting discovery. His findings led him to
hypothesize that elements combine at the atomic
level in fixed ratios. This ratio would naturally
differ in compounds due to the unique atomic
weights of the elements being combined.

11. This was a revolutionary idea but
further experimentation by himself and
others confirmed his theory. The
findings became the basis of of
Dalton’s Atomic Laws or Model. These
laws focus on five basic theorems.
First, Pure Elements consist of
particles called atoms. Second,atoms of
an element are all the same for that
element. That means gold is gold and
oxygen is oxygen down to the last atom.
Third, atoms of different elements can
be told apart by their atomic weights.
Fourth, atoms of elements unite to
form chemical compounds. Finally,
atoms can neither be created or
destroyed in chemical reaction. The
grouping only changes.

12. The last of Dalton’s Atom
Model were at the time
considered true for all reactions
involving atoms. This was later
corrected with the discovery of
nuclear fission and fusion. So we
now know that this only holds true
for chemical reactions.

13. Like other scientific theories,
Dalton’s atomic model has been
perfected over time with the research
and discoveries of other scientists. We
now know that the atom can be divided
into even smaller particles and we have
even discovered the actual internal
atom structure, even able to view it
through modern technology. We now
know that atomic weight is a product of
the structure of the atoms as well.

14. This atomic theory made possible modern
chemistry and physics. Up until Dalton’s time the
atom was only considered to a philosophical
construct passed down by the ancient Greeks.
Dalton’s ground breaking work made theory
reality. This understanding the atom helped to
fuel many other discoveries such as the
fundamental forces and Einstein’s theory of
relativity. It is especially is important when one
goes into Quantum physics a discipline that looks
at physics at the atomic and subatomic levels.

16. "A series of Essays read before this society and
afterwards published in the 5th Vol. of their Memoirs
gradually led me to the consideration of ultimate
particles or atoms & their combinations. Under the date
of September 3rd, 1803, I find in my notebook
'Observations on the Ultimate Particles of Bodies and
their Combinations,' in which the atomic symbols I still
use [were] introduced. On the 23rd of October the same
year[I] [r]ead my Essay on the absorption of gases [by
water] at the conclusion of which a series of atomic
[weights] was given for 21 simple and compound
elements .. . . "
- John Dalton (Excerpt from paper read to Manchester
Literary and Philosophical Society, 1830)