Atoms: The Building Blocks of Matter

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Atoms The Building Blocks of Matter

• The History of Atomic Theory

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Atomic Models

• This model of the atom may look familiar to you.
  This is the Bohr model. In this model, the
  nucleus is orbited by electrons, which are in
  different energy levels.
• A model uses familiar ideas to explain unfamiliar
  facts observed in nature.
• A model can be changed as new information is
  collected.

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• The atomic model has changed throughout the
  centuries, starting in 400 BC, when it looked
  like a billiard ball ?

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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
400 BC

• 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?

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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.

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The word atom comes from the Greek word
atomos that means unable to be cut
and you kept on cutting the leftover piece in
half
Imagine you had a piece of gold that you then cut
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
and kept going
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

• 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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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. Their
ideas held sway because of their eminence as
philosophers. The atomos idea was buried for
approximately 2000 years.
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Daltons Model

• 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

• He deduced that 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.

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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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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

• Thomson studied the passage of an electric
  current through a gas.
• As the current passed through the gas, it gave
  off rays of negatively charged particles.

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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 particles
  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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• Rutherfords experiment Involved firing a stream
  of tiny positively charged particles at a thin
  sheet of gold foil (2000 atoms thick)

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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,
  however, did bounce away from the gold sheet as
  if they had hit something solid. He knew that
  positive charges repel positive charges.

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An Animation of the Gold Foil Experiment

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

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• This could only mean that the gold atoms in the
  sheet were mostly open space. Atoms were 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

• Rutherford reasoned that all of an atoms
  positively charged particles were contained in
  the nucleus. The negatively charged particles
  were scattered outside the nucleus around the
  atoms edge.

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Bohrs Model

• In 1913, the Danish scientist Niels Bohr proposed
  an improvement.
• By studying what happens to atoms when they have
  gained energy and then lose it, he was able to
  propose that each electron exists in a specific
  energy level.

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The Hydrogen-Atom Line-Emission Spectrum

• The lowest energy state of an atom is its ground
  state.
• A state in which an atom has a higher potential
  energy than it has in its ground state (energy
  has been added) is an excited state.

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The Hydrogen-Atom Line-Emission Spectrum,
continued

• When investigators passed electric current
  through a vacuum tube containing hydrogen gas at
  low pressure, they observed the emission of a
  characteristic pinkish glow.
• When a narrow beam of the emitted light was
  shined through a prism, it was separated into
  four specific colors of the visible spectrum.
• The four bands of light were part of what is
  known as hydrogens line-emission spectrum.

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Hydrogen Line Emission Spectrum
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Hydrogen Line Emission Spectrum
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Emission and Absorption Spectrums

• When an electron falls to a lower energy level, a
  photon is emitted, and the process is called
  emission.
• Energy must be added to an atom in order to move
  an electron from a lower energy level to a higher
  energy level. This process is called absorption.

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Emission and Absorption Spectrums
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Bohr Model

• Niels Bohr proposed a hydrogen-atom model that
  linked the atoms electron to photon emission.
• According to the model, the electron can circle
  the nucleus only in allowed paths, or orbits.
  Sort of like the planets around the sun.
• The energy of the electron is higher when the
  electron is in orbits that are successively
  farther from the nucleus.

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Modern Quantum Model
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The Modern 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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Electrons as Waves

• French scientist Louis de Broglie suggested that
  electrons be considered waves confined to the
  space around an atomic nucleus.
• It followed that the electron waves could exist
  only at specific frequencies.
• According to the relationship E h?, these
  frequencies corresponded to specific energiesthe
  quantized energies of Bohrs orbits.

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Electrons as Waves, continued

• Electrons, like light waves, can be bent, or
  diffracted.
• Diffraction refers to the bending of a wave as it
  passes by the edge of an object or through a
  small opening.
• Electron beams, like waves, can interfere with
  each other.
• Interference occurs when waves overlap.

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Electrons as Waves
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The Heisenberg Uncertainty Principle

• German physicist Werner Heisenberg proposed that
  any attempt to locate a specific electron with a
  photon knocks the electron off its course.
• The Heisenberg uncertainty principle states that
  it is impossible to determine simultaneously both
  the position and velocity of an electron or any
  other particle.

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The Heisenberg Uncertainty Principle
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The Schrödinger Wave Equation

• In 1926, Austrian physicist Erwin Schrödinger
  developed an equation that treated electrons in
  atoms as waves.
• Together with the Heisenberg uncertainty
  principle, the Schrödinger wave equation laid the
  foundation for modern quantum theory.
• Quantum theory describes mathematically the wave
  properties of electrons and other very small
  particles.

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The Modern Quantum Model

• Since 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.

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Electron Cloud

• Is made up of areas known as orbitals. An orbital
  is a three-dimensional region around the nucleus
  that indicates the probable location of an
  electron.
• Electrons whirl about the nucleus billions of
  times in one second.
• They are not moving around in random patterns,
  but are moving around in a certain orbital.
• Location of electrons depends upon how much
  energy the electron has.

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Different Orbitals
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Electron Cloud

• Depending on their energy, electrons are locked
  into a certain orbital 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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Indivisible Electron Nucleus Orbit Electron Cloud
Greek X
Dalton X
Thomson X
Rutherford X X
Bohr X X X
Modern X X X
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The Different Models