Text of Development of Atomic Theory Part I. Early Roots Much of science (including chemistry) and...
Development of Atomic Theory Part I
Early Roots Much of science (including chemistry) and
mathematics can trace their investigative roots to Arabic countries
in the Middle Ages and their philosophical roots to ancient Greece.
It was in the Middle and Dark Ages that alchemy as a quasi-science
came into practice. As many know, alchemy was the quest to turn
common elements (such as lead) into gold-a precious metal. As we
hopefully know by now, a transmutation such as this would not occur
in a chemical reaction. By the late 1700s most chemists and
scientists had accepted a definition of an element as a substance
that cannot be further broken down by ordinary chemical means. They
also knew that elements combine to form new compounds that have
different physical and chemical properties from the elements that
formed them. There was still much to know, such as did elements
always combine in the same ratio when forming the same
First Laws Once scientific methods had developed to the point
where quantitative data was being recorded (particularly mass), our
understanding of elements and their chemical ratio behavior was
furthered. This occurred in the late 1700s and early 1800s. One of
the most basic laws that serves as the foundation of chemistry, the
Law of Conservation of Mass, came about during this time. The Law
of Conservation of Mass states that mass is neither created nor
destroyed during ordinary chemical reactions or physical
First Laws (cont) Soon thereafter, the Law of Definite
Proportions was proposed and supported. This law states that a
chemical compound contains the same elements in exactly the same
proportions by mass regardless of the size of the sample or source
of the compound. In other words, 18.02 g of water (H 2 O) always
contains 2.02 g of hydrogen and 16.00 g of oxygen. Continuing the
example, 36.04 g of water always contains 4.04 g of hydrogen and
32.00 g of oxygen. These ratios are constant (definite). This is
also known as the Law of Constant Composition.
First Laws (cont) The next chemical law that became accepted
was the Law of Multiple Proportions. It states that if two or more
different compounds are composed of the same two elements, then the
ratio of the masses of the second element combined with a certain
mass of the first is always a ratio of small whole numbers.
First Laws (cont) Lets explain with methane (CH 4 ) and ethane
(C 2 H 6 ) If I have 16.05 g of methane then I know from the Law of
Definite Proportions that I have 12.01 g of C and 4.04 g of H. When
I compare that to a sample that has the same number of ethane
particles as the methane did, then it would mass 30.08 g of C 2 H
6. It would contain 24.02 g of C and 6.06 g of H. The Law of
Multiple Proportions allows me to compare the C from the methane to
the C from the ethane. 12.01 g: 24.02 g or a 1:2 ratio. The H ratio
(again methane to ethane) would be 4.04 g: 6.06g or a 2:3 ratio, as
they are both reduce when rounded. Essentially the Law of Multiple
Proportions says that there is some whole number ratio that relates
the same element in two different compounds. We did not innately
understand the significance of this fact, but it was one of those
odd things that scientists knew would reveal new ideas.
Dalton During the turn of the century (1800s-1900s), John
Dalton, an English school teacher, proposed the idea of the atom as
scientific fact and not esoteric philosophy, and offered these
Dalton Daltons Atomic Theory 1. All elements are composed of
tiny, indivisible particles called atoms. 2. Atoms of the same
element are identical. The atoms of any one element are different
from those of any other element. 3. Atoms of different elements can
combine with one another in simple whole number ratios to form
compounds. 4. Chemical reactions occur when atoms are separated,
joined, or rearranged. Atoms of one element cannot be changed into
atoms of another element by a chemical reaction.
Dalton If you study Daltons Atomic Theory, you should first
notice how the Laws of Conservation of Mass, Definite Proportions,
and Multiple Proportions all came together and were used to explain
the existence and role of atoms of elements. Some of his ideas have
had to be further refined and updated.
Dalton As science moved forward (and we will get into that
later in the lesson), precepts 1 and 2 had to changed. While the
atom is still the smallest whole unit of an element, we now know
that it is itself comprised of even smaller particles (protons,
neutrons, electrons, etc.). Also, we also know that there can be
different types of atoms of the same element because they can
differ by their neutron count and still be the same atom, because
their proton and electron count is still the same. The isotope is
the term we have given to atoms of the same element that differ by
their neutron count. But Daltons Atomic Theory was the first that
gave concrete evidential support to the existence of the atom.
Thomson Joseph John Thomson, researching in the 1890s, realized
that the accepted model of an indivisible atom did not take
electrons and protons into account.
Thomson His work with the cathode tube ray showed that there
were regions of positive and negative charges in the atom. Thomson
is credited with discovering the electron (1897).
Thomson His atomic model is often called the Plum Pudding
Model. (Chocolate-chip cookie dough is a good analogy).
Thomson The negatively charged electrons (the chocolate chips)
are stuck into a lump of positively charged protons (the dough). He
was awarded the Nobel Prize in Physics in 1906.
Thomson Limitations: His model still does not include neutrons.
This model explained some of the electrical properties of the atom,
but said nothing about the number of protons or their arrangement
in the atom, or how easily atoms are stripped of their electrons to
Rutherford Ernest Rutherford discovered the proton and the
nucleus in the early 1900s Proposed the nuclear atom in which
electrons surround a dense nucleus comprised of protons and
neutrons. He thought of the rest of the atom as empty space. Other
work showed that the nuclei of most atoms are composed of protons
Rutherford His experiment is known as the gold foil experiment.
Rutherford This work was performed over 1906-1907. He was
awarded the Nobel Prize in Chemistry in 1908.
Rutherford This model, though, does not account for the fact
that opposite particles attract each other. If this model is
correct, the electrons in the empty space around the nucleus in the
atom should be drawn into the nucleus due to the attraction to the
Chadwick James Chadwick worked for Rutherford in Cambridge,
England. In 1932, he finds a subatomic particle that has no charge
while studying uranium-235 decay. The particle is eventually named
the neutron as it had no charge.
Chadwick He won the Nobel Prize in Physics in 1935.