GO 3
Describe ideas used in interpreting the chemical nature of matter, both
in the past and present, and identify example evidence that has
contributed to the development of these ideas
3-1 Evolution of the Atomic Theory
Guided Question
How did our understanding of the Atom
evolve?
Introduction
Atomic theory first originated with Greek philosophers
around 2500 years ago. This basic theory remained
unchanged until the 19th century when it first became
possible to test the theory with more sophisticated
experiments.
As science has rapidly advanced over the past few
centuries the atomic theory has been refined in
accordance with the accepted scientific principles and
theories of the time.
Ancient Atomic Theory
The atomic theory of matter was first proposed by
Leucippus, a Greek philosopher who lived at around
400BC. At this time the Greeks were trying to
understand the way matter is made.
According to Leucippus, eventually you arrive at
small particles which can not be further subdivided.
Leucippus called these indivisible particles atoms
(from the Greek word atomos, meaning “indivisible”).
Leucippus
Ancient Atomic Theory continued..
Leucippus's atomic theory was further developed by his
disciple, Democritus who concluded that infinite
divisibility of a substance belongs only in the imaginary
world of mathematics.
Democritus
Democritus suggested the atomic theory, explaining
that all things are "composed of minute, invisible,
indestructible particles of pure matter which move
about eternally in infinite empty.". If a sample of a pure
element was divided into smaller and smaller parts,
eventually a point would be reached at which no
further cutting would be possible—this was the atom of
that element.
According to the ancient Greeks, atoms were all made of the same basic
material, but atoms of different elements had different sizes and shapes. The
sizes, shapes, and arrangements of a material’s atoms determined the
material’s properties.
ARISTOTLE:
Everything is made of Four ELEMENTS:
The Modern Atomic Theory
For centuries scientists did not have the methods or technology to test
their theories about the basic structure of matter, so people accepted the
ancient Greek view.
In the 19th century John Dalton made inferences that
exhibited how atoms bond together in definite
proportions. Dalton was able to say that atoms of
different elements combine in whole number ratios.
John Dalton
This theory, to go along with four other theories, made
up what Dalton called the "Modern Atomic Theory.“
Included in these were two theories that stated atoms
could not be divided or destroyed, a theory that stated
different elements contain different chemical properties,
and atoms of the same element contain the same
chemical properties
The Modern Atomic Theory
continued…
Dalton made two assertions about atoms: (1) atoms of each
element are all identical to one another but different from the
atoms of all other elements, and (2) atoms of different
elements can combine to form more complex substances.
Although the two theories that
speculated atoms couldn't be divided
were false, Dalton contributed greatly
to the advances of atomic theory, and
would greatly influence J.J. Thompson
in his own discoveries.
Billiard Ball Model
Expanding the Modern Atomic Theory
J.J. Thomson
J.J. Thomson is the person who is credited for discovering
the electron. Thompson created a tube that had a positively
charged anode on one side and a negatively charged
cathode on the other side. Thomson then applied a magnet
to the middle of the tube and discovered that negatively
charged particles were emanating towards the positive
magnetic field. From this, Thomson concluded that
negatively charged particles, called electrons, were present
in atoms.
Thomson then created the Plum Pudding
model, which suggested that electrons
and protons were randomly placed
throughout the atom.
The “Plum Pudding”
Rutherford's Experiment
In 1911 British scientist Ernest Rutherford set
out to test Thomson’s proposal by firing a beam
of charged particles at atoms.
Alpha particles are heavy particles with twice
the positive charge of a proton. Alpha particles
are now known to be the nuclei of helium
atoms, which contain two protons and two
neutrons.
Ernest Rutherford
Ernest Rutherford's experiment was to emit
alpha particles towards a thin gold sheet.
Rutherford would then determine where the
deflections of the alpha particles would go, and
therefore be able to theorize what kind of
placement protons and electrons had.
Rutherford's Experiment continued…
Rutherford's Experiment continued…
Rutherford observed that most of the alpha particles went strait through the
foil. However a large proportion were deflected through small angles an some
(though very few) deflected straight back.
Rutherford then theorized that there was something called a nucleus, which
contained a high density of positively charged particles. Rutherford was able
to say there was a nucleus because alpha particles that deflected right back
must have hit something more massive and with a strong positive charge. This
led Rutherford to propose a very different model for the atom.
Instead of supposing that the positive charge and mass
were spread throughout the volume of the atom, he
theorized that it was concentrated in the center of the
atom. Rutherford called this concentrated region of
electric charge the nucleus of the atom.
Bohr’s Model
Danish physicist Niels Bohr used new knowledge about
the radiation emitted from atoms to develop a model of
the atom significantly different from Rutherford’s model.
Niels Bohr
Bohr developed a theory by which he could predict the
same wavelengths scientists had measured radiating from
atoms with a single electron. He concluded that because
atoms emit light only at discrete wavelengths, electrons
could only orbit at certain designated radii, and light could
be emitted only when an electron jumped from one of
these designated orbits to another.
BOHR’s Model
2+ 8 + 8 electrons
Ar
18p+
18no
3 electron shells
Quantum Theory of the Atom
To make his theory work, Bohr had to propose special rules that violated
the rules of classical physics. He concluded that, on the atomic scale,
certain preferred states of motion were especially stable. In these states of
motion an orbiting electron (contrary to the laws of electromagnetism)
would not radiate energy.
The quantum mechanical view of atomic
structure is that the nucleus is at the center
of the atom and provides the electrical
attraction that binds the electrons to the
atom. Contrary to Bohr’s theory, however,
the electrons do not circulate in definite
planet-like orbits. Due to the wavelike
character of electrons and provides the
framework for viewing the electrons as
fuzzy clouds of negative charge.
So what does it all mean?
• An atom is made up of 3 main parts:
• Protons (p+)– positive particle found in the
Nucleus of the atom
• Neutrons (n 0)- neutral particles also found in
the middle of the atom
• Electron (e-) – negatively charged particles
found orbiting the protons and neutrons (the
Nucleus)
Electron Knowledge!
Electrons always fill in the First electron shell
first
Stationary state – the most stable state of an
atom, electrons are “at rest” (moves in
circular orbit so electromagnetic energy
remains constant.
valence electrons – the electrons in the Last
electron shell
Nucleus
• Protons and Neutrons combine to form the
Nucleus.
• The electrons orbit in circles called ELECTRON
SHELLS.
Electrons and Shells?
• Electrons are found orbiting the nucleus in a
series of electron shells (a.k.a. energy levels )
• Higher energy levels are further away from
the nucleus
• Lower energy levels are closer to the nucleus
• The pattern is : 2 e-, 8 e-, 8 e- ,16 e-, 16 e-, 32 e-
What is in an atomic number?
The atomic number tells you the number
of Protons in an atom’s nucleus
If you change the number of protons,
you change the ELEMENT!
• For an Element,
# of protons (p+) = # of electrons (e-)
So, their charges are NEUTRAL
What about the neutrons?
Neutrons are within the atom’s nucleus and have no
electrical charge !!
(Their symbol is: no)
• Again…neutrons and protons give an atom its
mass, so to calculate number of neutrons in
an atom:
Atomic mass
of
an atom
Subtract
# of protons in
element (atomic #)
References
•encarta.msn.com/encyclopedia
•www.funsci.com/fun3_en/democritus/democritus
•www.aare.edu.au
•www.absoluteastronomy.com/encyclopedia