Download Atomic Structure History 1213

The History of Atomic
Structure
AKA: The old, dead guy parade!
CA State Standards: 1h and 1i
Democritus (460 - 370 BC)
Was the first person to come
up with the idea of atom
Believed that all matter was
composed of indivisible
particles he called “ATOMS”
Was he right? Are atoms
indivisible??
He also believed that different
atoms:
Are different sizes
Have different properties
Objectives
To identify the major scientists and
what they contributed to our knowledge
of atomic structure
To describe the evolution of our
understanding of atomic structure
To describe Rutherford’s Gold Foil
experiment and what it concluded
John Dalton (1766-1844)
• Dalton is the “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.
Dalton’s Atomic Theory (1803)
Dalton’s Atomic Theory (1803)
aka: 5 Postulates
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. (a Sodium atom is different than
chlorine)
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. We will learn more later**
5. Law of Definite Proportions: Different
atoms combine in simple whole
number ratios to form compounds
(you can’t have a ½ of a Carbon bonding with
Oxygen; it’s a whole atom or no atom)
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JJ Thomson (1856-1940)
Solid Ball Model
Dalton’s TinyAtomic
•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.
• Used cathode rays to prove
that Dalton’s Solid-ball model
could be broken into smaller
particles
•(Sorry Dalton! No more tiny
SOLID balls!)
•Thomson is credited with
discovering electrons
•Dalton was credited for the
three Atomic Laws that were
proven after his time.
•Milikan confirmed
Thomson’s work with the oil
droplet experiment.
Cathode Ray Tubes
• Cathode rays had been used for
Cathode +
some time before Thompson’s
experiments.
Rutherford (1871-1937)
Anode • 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 negatively charged
particles. They travel from cathode, across the tube to the
anode.
Gold Foil Experiment
Rutherford directed a narrow
beam of alpha particles (+
charges) at a thin piece of
gold foil
Based on the Plum Pudding
Model, he predicted that the
+ charges would go through
the foil
Because the atom overall was
neutral, he thought the alpha
particles would travel straight
through
Took Thomson’s Plum
Pudding Model and
added to it
“Gold Foil Experiment”
in 1913 was used to
discover the existence:
Protons
Nucleus
You must be able to
explain the Gold Foil
Experiment…it will be on
the CST
Bohr (1885-1962)
Worked in Rutherford’s lab
Wondered why – electrons
are not attracted to the +
nucleus and cluster around it
Disproved Thomson’s Plum
Pudding Model
Used Rutherford’s Planetary
Model and Einstein’s photon
discovery to propose a new
model
Bohr said that electrons travel
at the speed of light.
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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
nucleus at the speed of light.
•Electrons can “jump” from one
circle (orbit) to the next
•But they are not attracted to
the nucleus they traveling too
fast to be fully attracted.
Bohr’s Energy Level Model
He proposed the following:
1. Protons and neutrons are in the nucleus
2. Electrons can only be found at 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
Bohr’s & Energy Level Model Video
Electron Cloud Model
• Bohr thought that electrons traveled in a perfect circle
every revolution. (circumference)
• We know that the orbits of electrons are not that perfect.
• Electrons have a bit of wobble in their travel, similar to the
path of a runner on a track. Electrons can be anywhere in
their lane. The lanes are called electron clouds.
• The Electron Cloud Model improved upon Bohr’s model by
using mathematics to determine the position of an electron
in an orbit (lane).
•An electron will be found in it’s cloud about 90% of the
time. The picture above shows all of the places an electron
can be found in an atom.
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