Download Atomic Theory Notes

History of the Atom - Timeline
1766 – 1844
1887 – 1961
James
Chadwick
discovered
the neutron
in 1932
460 – 370 BC
0
Scientists and Their Contribution to
the Model of an Atom
Niels Bohr
proposes
the Bohr
Model in
1913
1700s
1800s
1900s
History of the Atom
Antoine Lavoisier
Thomson
makesJ.J.
a substantial
the
number discovers
of contributions
electron
and
to the
field of
proposes the
Chemistry
Plum Pudding
Model 1871
in 1897
– 1937
Democritus
proposes
the 1st atomic
theory
1743 – 1794
Erwin
John Dalton
Ernest Rutherford
Schrodinger
proposesperforms
his
the Gold Foil
describes
1891 – 1974
atomic theory
Experiment
in
in 1909
the electron
1803
cloud in 1926
1885 – 1962
Click on picture for more information
1856 – 1940
•  Believed that all matter was
composed of indivisible
particles he called
“ATOMS”
Democritus
(460 BC – 370 BC)
•  Proposed an Atomic Theory
which states that all atoms
are small, hard, indivisible and
indestructible particles made
of a single material formed
into different shapes and
sizes.
•  Aristotle did not support his
Atomic Theory
•  Derived from the Greek word
“Atomos” – meaning
indivisible
•  He also believed that different
atoms:
Image taken from:
https://reichchemistry.wikispaces.com/T.+Glenn
+Time+Line+Project
–  Are different sizes
–  Have different properties
1
Antoine Lavoisier
(1743 – 1794)
Image taken from:
www.ldeo.columbia.edu/.../v1001/
geotime2.html
•  Known as the “Father of Modern
Chemistry”
•  Was the first person to generate a list of
thirty-three elements in his textbook
•  Devised the metric system
•  Was married to a 13-year old Marie-Anne
Pierette Paulze; she assisted him with
much of his work
•  Was a tax-collector that was
consequently guillotined during the
French Revolution
•  Discovered/proposed that combustion
occurs when oxygen combines with other
elements
•  Discovered/proposed the Law of
Conservation of Mass (or Matter) which
states, in a chemical reaction, matter is
neither created nor destroyed
“Father of Atomic Theory”
Dalton’s ideas were
so brilliant that they
have remained
essentially intact up
to the present time
and has only been
slightly corrected.
John Dalton
(1766 – 1844)
•  In 1803, proposed an Atomic Theory
which states:
o  All substances are made of atoms;
atoms are small particles that
cannot be created, divided, or
destroyed.
o  Atoms of the same element are
exactly alike, and atoms of
different elements are different
o  Atoms join with other atoms to
make new substances
•  Calculated the atomic weights of many
various elements
•  Was a teacher at a very young age
•  Was color blind
Image taken from:
chemistry.about.com/.../JohnDalton.htm
Dalton’s Atomic Theory (1803)
aka: 5 Postulates
1.  All matter is composed of extremely small
particles called atoms. (I agree with Democritus!)
2. All atoms of a given element are
identical, having the same:
- size
- mass
- chemical properties.
3. All atoms of different elements are
different.
2
Dalton’s Atomic Model
Dalton’s Atomic Theory (1803)
aka: 5 Postulates
4.  Atoms cannot be created, divided into
smaller particles, or destroyed.
**In a chemical reaction, atoms of different elements
are separated, joined or rearranged. They are
never changed into the atoms of another element.**
5. Atoms combine in definite whole
number ratios to make compounds
(you can’t have a ½ of a Carbon bonding with
Oxygen; it’s a whole atom or no atom)
• Based on Dalton’s Atomic
Theory (5 postulates), most
scientists in the 1800s believed
that the atom was like a tiny
solid ball that could not be
broken up into parts.
• Dalton was credited for the
three Atomic Laws that were
proven after his time.
Dalton’s Atomic Laws
1. Law of Conservation of Mass
Matter cannot be created or
destroyed in any physical or
chemical process, just transferred.
2.
Dalton’s Atomic Laws
3. Law of Multiple Proportions – if two
elements can combine to form more than one
compound, then the ratio of the second element
combined with a certain mass of the first element
is always a ratio of small whole numbers.
Law of Constant Composition
When atoms combine to form molecules,
the ratio of atoms is constant.
Example – H2O will always have 2 times
as many Hydrogen atoms as Oxygen.
Example:
CO vs. CO2
Formula
Ratio of N:O
3
J.J. Thomson
(1856 – 1940)
Image taken from:
www.wired.com/.../news/2008/04/
dayintech_0430
•  Proved that an atom can be divided
into smaller parts
•  While experimenting with cathoderay tubes, discovered corpuscles,
which were later called electrons
•  Stated that the atom is neutral
•  In 1897, proposed the Plum Pudding
Model which states that atoms
mostly consist of positively charged
material with negatively charged
particles (electrons) located
throughout the positive material
•  Won a Nobel Prize
Cathode Ray Tubes
•  Cathode rays had been used for
some time before Thompson’s
experiments.
•  A cathode ray is a tube that has
a piece of metal, called an
electrode, at each end. Each
electrode is connected to a power
source (battery).
•  When the power is turned on, the electrodes
become charged and produce a stream of charged
particles. They travel from cathode, across the tube
to the anode.
•  Used cathode rays to
prove that Dalton’s
Solid-ball model could be
broken into smaller
particles
• Thomson is credited
with discovering
electrons
Cathode Ray Tubes
•  Thomson put the tube
in a magnetic field.
He predicted that the
stream would travel in
a straight path.
•  Instead, he found that
the path curved away
from a negatively
charged plate and
toward a positively
charged plate
•  Why?
-----------------------------
++++++++++++++
• Like charges repel each other,
and objects with unlike charges
attract each other, Thomson
concluded that the stream of
charged particles had electrons
in them.
4
Cathode Ray Tube Experiment
• Thomson’s Plum Pudding model is a +
charged sphere that has (-) charged
electrons scattered inside, like “raisins” in
“plum pudding.”
• Overall, the atom is neutral because the
atom had the same number of positive
and negative charges.
• Thompson Concluded:
• Cathode rays are made up of
invisible, negatively charged
particles called Electrons.
• These electrons had to come from the matter (atoms)
of the negative electrode.
• Since the electrodes could be made from a variety of
metals, then all atoms must contain electrons!
Ernest Rutherford
(1871 – 1937)
•  In 1909, performed the
Gold Foil Experiment and suggested the
following characteristics of the atom:
o  It consists of a small core, or nucleus,
that contains most of the mass of the
atom
o  This nucleus is made up of particles
called protons, which have a positive
charge
o  The protons are surrounded by
negatively charged electrons, but most
of the atom is actually empty space
•  Did extensive work on radioactivity (alpha
& beta particles, gamma rays/waves) and
was referred to as the “Father of Nuclear
Physics”
•  Won a Nobel Prize
•  Was a student of J.J. Thomson
•  Was on the New Zealand $100 bill
Thomson’s Plum Pudding Model
• From Thomson’s experiments, scientists concluded that
atoms were not just neutral spheres, but somehow were
composed of electrically charged particles.
• The balance of positive and negative charge supports the
neutral atom.
Rutherford (1871-1937)
•  Used the “Gold Foil
Experiment” to discover
the existence of:
–  An atomic Nucleus
–  Protons (in later experiments)
Image taken from:
http://www.scientific-web.com/en/
Physics/Biographies/
ErnestRutherford.html
5
Gold Foil Experiment
•  Rutherford directed a narrow
beam of alpha particles (+
charges) at a thin piece of
gold foil.
•  Based on observations from
other experiments involving
alpha particles, he predicted
that the (+) charges would
go through the foil
The Gold Foil Experiment
Results from Gold Foil Experiment
• Rutherford found that every
once and a while, a + particle
was deflected bounced back.
(about 1% of the time)
• Why?
• Because the + charge hit a
central mass of positive charge
and was repelled.
Conclusions from Rutherford’s
Gold Foil Experiment
•  The atom contains a positively charged
“nucleus”
• This nucleus contains almost all of the mass of
the atom, but occupies a very small volume of
the atom.
• The negatively charged electrons occupied most
of the volume of the atom.
•  The atom is mostly empty space.
6
Niels Bohr
Rutherford’s Planetary Model
(1885 – 1962)
Image taken from:
commons.wikimedia.org/wiki/
File:Niels_Bohr.jpg
The electrons orbit the nucleus like the
planets revolve around the sun.
Bohr (1885-1962)
•  Worked in Rutherford’s lab
•  Wondered why – electrons
are not attracted to the +
nucleus and cluster around
it
•  Disproved Rutherford’s
Planetary Model
•  Experimented with light and
its interaction with matter
to develop a new model.
•  In 1913, proposed the Bohr
Model, which suggests that
electrons travel around the
nucleus of an atom in orbits or
definite paths. Additionally, the
electrons can jump from a path
in one level to a path in another
level (depending on their energy)
•  Won a Nobel Prize
•  Worked with Ernest Rutherford
Bohr’s Energy Level Model
Energy Level Model: Electrons are
arranged in circles around the
nucleus. Each circle has a different
energy.
• Electrons are in constant motion,
traveling around the circle at the
speed of light.
n 
• Electrons can “jump” from one circle
to the next
• But they can’t go to the nucleus they are traveling too fast to be fully
attracted.
7
Bohr’s Energy Level Model
He proposed the following:
1.  Protons and neutrons are in the nucleus
2.  Electrons can only be certain distances from
the nucleus.
3.  The electrons orbit the nucleus at fixed
energy levels.
4.  The electrons must absorb or emit a fixed
amount of energy to travel between these
energy levels
James Chadwick
(1891 – 1974)
Erwin Schrodinger
(1887-1961)
•  In 1926, he further explained
the nature of electrons in an
atom by stating that the exact
location of an electron cannot be
stated; therefore, it is more
accurate to view the electrons in
regions called electron clouds;
electron clouds are places where
the electrons are likely to be
found
•  Did extensive work on the Wave
formula à Schrodinger equation
•  Won a Nobel Prize
Image taken from:
nobelprize.org/.../1933/
schrodinger-bio.html
Wave Model
•  Realized that the atomic mass of
most elements was double the
number of protons à discovery
of the neutron in 1932
•  Worked on the Manhattan
Project
•  Worked with Ernest Rutherford
•  Won a Nobel Prize
Image taken from:
www.wired.com/.../news/2009/02/
dayintech_0227
8
The Wave Model
•  Today’s atomic model is
based on the principles
of wave mechanics.
•  According to the theory
of wave mechanics,
electrons do not move
about an atom in a
definite path, like the
planets around the sun.
Electron Cloud:
•  A space in which
electrons are likely to be
found.
•  Electrons whirl about the
nucleus billions of times
in one second
•  They are not moving
around in random
patterns.
•  Location of electrons
depends upon how much
energy the electron has.
The Wave Model
•  In fact, it is impossible to determine the exact
location of an electron. The probable location of
an electron is based on how much energy the
electron has.
•  According to the modern atomic model, at atom
has a small positively charged nucleus
surrounded by a large region in which there are
enough electrons to make an atom neutral.
Electron Cloud:
•  Depending on their energy they are locked into a
certain area in the cloud.
•  Electrons with the lowest energy are found in
the energy level closest to the nucleus
•  Electrons with the highest energy are found
in the outermost energy levels, farther from
the nucleus.
9
Indivisible Electron
Greek
X
Dalton
X
Thomson
Nucleus
Orbit
Electron
Cloud
Progression of the Atomic Model
-
-
X
Rutherford
X
X
Bohr
X
X
Wave
X
X
-
X
- --+-
-
+
- -
Electron Cloud
X
The structure of an atom, according to:
Democritus
James
Ernest
Erwin
Neils
Schrodinger
Chadwick
Rutherford
Bohr&
J.J.
Thomson
John Dalton
10