Chapter 4 Section 1 Notes

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Chapter 4 Section 1Notes

• The History of Atomic Theory

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Who are these men?
In this lesson, well learn about the men whose
quests for knowledge about the fundamental nature
of the universe helped define our views.
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Democritus
460-370 B.C.E.

• This is the Greek philosopher Democritus who
  began the search for a description of matter more
  than 2400 years ago.
• He asked Could matter be divided into smaller
  and smaller pieces forever, or was there a limit
  to the number of times a piece of matter could be
  divided?
• He had no way of knowing what atoms looked like!

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The word atom comes from a Greek word that
means unable to be cut
Imagine you had a piece of gold that you then cut
in half
and you kept on cutting the leftover piece in
half
and then you cut one of these smaller pieces in
half
Gold
Go
ld
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The word atom comes from a Greek word that
means unable to be cut
Eventually you would have 1 piece of gold left.
If you cut it in half, you wouldnt have gold any
more youd have something else. This tiny,
tiny single piece of gold is called an atom of
gold. An atom is the smallest particle of an
element that acts like the element.
and kept going
and kept going
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Atomos

• His theory Matter could not be divided into
  smaller and smaller pieces forever, eventually
  the smallest possible piece would be obtained.
• This piece would be indivisible.
• He named the smallest piece of matter atomos,
  meaning not to be cut.

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Atomos

• To Democritus, atoms were small, hard particles
  that were all made of the same material but were
  different shapes and sizes.
• Atoms were infinite in number, always moving and
  capable of joining together.

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• This theory was ignored and forgotten for more
  than 2000 years!

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A Big Debate

• Can matter be divided into smaller and smaller
  pieces forever?

What do you think?
YES!
NO!
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Democritus

• He hypothesized that atoms were
• Small Hard
• Different in shape size
• Infinite
• Always moving
• Capable of joining

But no one believed him!
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Why?

• The eminent philosophers of the time, Aristotle
  and Plato, had a more respected, (and ultimately
  wrong) theory.

Aristotle and Plato favored the earth, fire, air
and water approach to the nature of matter. The
atomos idea was buried for approximately 2000
years.
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Time Goes By

• 1600s-1700s Key experiments occur which support
  Democrituss ideas.
• Robert Boyle (1627-1691)
• Antoine Lavoisier (1743-1794)

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Dalton (1766-1844) Daltons Model

• English chemist and school teacher
• In the early 1800s, the English Chemist John
  Dalton performed a number of experiments that
  eventually led to the acceptance of the idea of
  atoms.

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Daltons Theory (1803)

• All elements are composed of atoms.
• Atoms are indivisible and indestructible
  particles.
• Atoms of the same element are exactly alike.
• Atoms of different elements are different.
• Compounds are formed by the joining of atoms of
  two or more elements.

His model was called the Billiard Ball!
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.

• This theory became one of the foundations of
  modern chemistry.

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Thomsons Plum Pudding Model

• In 1897, the English scientist J.J. Thomson
  provided the first hint that an atom is made of
  even smaller particles.

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Atomic Model Thomson

• Passed electricity through an uncharged gas
• The gas gave off rays to show it was NEGATIVELY
  charged
• How?
• Negative charges must come from inside the atom!
• gt Electrons!

Click here for more on the Discovery of the
electron
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(No Transcript)
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But wait

• How can an atom be NEUTRAL if it is full of
  negatively charged particles (electrons?)

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Atomic Models Thomson

• The atoms are neutral How?
• charges must be present to balance - charges
• - lumped in a cluster he said looked like
  plum pudding

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Thomson Model

• He proposed a model of the atom that is sometimes
  called the Plum Pudding model.
• Atoms were made from a positively charged
  substance with negatively charged electrons
  scattered about, like raisins in a pudding.

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Thomson Model
Where did they come from?

• This surprised Thomson, because the atoms of the
  gas were uncharged. Where had the negative
  charges come from?

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Thomson concluded that the negative charges came
from within the atom. A particle smaller than
an atom had to exist. The atom was divisible!

• Thomson called the negatively charged
  corpuscles, today known as electrons.
• Since the gas was known to be neutral, having no
  charge, he reasoned that there must be positively
  charged particles in the atom.
• But he could never find them.

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Rutherfords Gold Foil Experiment

• In 1908, the English physicist Ernest Rutherford
  was hard at work on an experiment that seemed to
  have little to do with unraveling the mysteries
  of the atomic structure.

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Atomic Models Rutherford

• Passed Charged Particles through gold foil
• Most passed right through
• Atom is mostly empty space
• Some bounced off at odd angles
• Nucleus must be
• Calculate size of nucleus

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Rutherfords Gold Foil Experiment

start experiment
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Rutherfords Work

What's Different This Time???
start experiment
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• Rutherfords experiment involved firing a stream
  of tiny positively charged particles at a thin
  sheet of gold foil (2000 atoms thick)

• http//chemmovies.unl.edu/ChemAnime/RUTHERFD/RUTHE
  RFD.html

• rutherford.mov

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• Most of the positively charged bullets passed
  right through the gold atoms in the sheet of gold
  foil without changing course at all.
• Some of the positively charged bullets, bounced
  away from the gold sheet as if they had hit
  something solid.
• He knew that positive charges repel positive
  charges.

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• This could only mean that the gold atoms were
  mostly open space, not a pudding filled with a
  positively charged material.
• Rutherford concluded that an atom had a small,
  dense, positively charged center that repelled
  his positively charged bullets.
• He called the center of the atom the nucleus
• The nucleus is tiny compared to the atom as a
  whole.

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Rutherford

• All of an atoms positively charged particles
  were contained in the nucleus.
• The negatively charged particles were scattered
  outside the nucleus.

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Okay

• So the atom is made up of positive and negative
  particles.
• Where are the electrons found in the atom?

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Bohr Model

• In 1913, the Danish scientist Niels Bohr proposed
  an improvement. In his model, he placed each
  electron in a specific energy level.

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Bohrs Model
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Bohrs Model

• According to Bohrs atomic model
• Electrons respond to light
• Electrons move in definite orbits around the
  nucleus
• Like planets around the sun
• Electrons are found in specific energy levels
• These orbits, or energy levels, are located at
  certain distances from the nucleus.
• Electrons can jump from path to path (like a
  ladder)

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The Electron Cloud Model (a.k.a. The Wave Model)

• Todays 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.

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Atomic Model Electron Cloud (wave)

• Electrons move so fast that it is impossible to
  determine their location
• Move in all directions around the nucleus

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Electron Cloud Model
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The Electron Cloud Model

• I 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.
• In the electron cloud model, the atom has a small
  positively charged nucleus surrounded by a large
  region in which there are enough electrons to
  make an atom neutral.

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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.

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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.

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Atomic Model Review
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Daltons Atomic Model(1807)

• The Billiard ball model.
• Atoms were considered to be featureless,
  indivisible, spheres of uniform density

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J. J. Thomsons Atomic Model Plum Pudding Model

• Atoms are not hard spherical balls.
• Further experiments showed two particles in atoms
• Protons () electrons (-)
• Electrons were like raisins stuck randomly inside
  a mass of positively charged pudding.
• Negative electrons used to hold together the
  positively changed mass of pudding.

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Rutherfords Nuclear Model(1911)

• Central nucleus composed of positively charged
  protons.
• Negatively charged cloud of electrons surrounds
  the nucleus.
• PROBLEM As atoms lost energy why didnt the
  electrons eventually spiral into the nucleus due
  to electrical attraction of for -?
• PROBLEM How could all the positive protons stay
  packed together in the nucleus?

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Neil Bohrs Model of Hydrogen(1913)

• Solves problem of why electrons to do fall into
  nucleus.
• Used quantized orbits with specific energies.
• Electron can only move between orbits by getting
  or losing the exact amount of energy required.
• It could not take fractional steps.

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Schrödinger Electron Cloud Model(1926)

• According to Schrödinger atoms are composed of
• Central nucleus
• A fuzzy electron cloud

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Summary of Atomic Models
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Indivisible Electron Nucleus Orbit Electron Cloud
Greek X
Dalton X
Thomson X
Rutherford X X
Bohr X X X
Electron Cloud X X X