2.1 Atoms and Their Structure

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2.1 Atoms and Their Structure

• Objective
• Relate historic experiments to the development of
  the modern model of the atom.
• Illustrate the modern model of the atom.
• Interpret the information available in an element
  block of the periodic table.

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Early Ideas About Matter

• Our current model of the composition of matter is
  based on hundreds of years of work.
• About 2500 years ago, the Greek philosophers
  thought about the nature of matter and its
  composition.
• They proposed that matter was composed of four
  fundamental elements

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And these were

• Air
• Water
• Fire
• Earth

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Greek Philosophers

• They also debated whether matter could be divided
  into smaller and smaller pieces endlessly or
  whether there was an ultimate smallest particle
  that could no longer be divided.
• They were excellent observers, but they did not
  test their theories with experiments.

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Democritus, 460-370 B.C.

• Proposed that the world is made up a empty space
  and tiny particles called atoms.
• He thought that atoms were the smallest particles
  of matter and that different types of atoms exist
  for every type of matter.
• Atomic Theory- matter is made up of fundamental
  particles called atoms.

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Antoine Lavoisier (1743-1794)

• French chemist
• In 1782 he made measurements of a chemical change
  in a sealed container.

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Lavoisiers Conclusion

• He observed that the mass of the reactants before
  the chemical reaction were equal to the mass of
  the products after the reaction.
• In a sealed container 2.0 g of hydrogen gas
  reacted with 16.0 g of oxygen gas to produce 18.0
  g of water
• Matter was neither created nor destroyed during a
  chemical reaction, but changed.
• This became known as the law of conservation of
  matter

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Law of Conservation of Mass
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Joseph Proust

• Joseph Proust, French chemist
• September 26, 1754 July 5, 1826

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Prousts Contribution

• Observed that the composition of water is always
  11 hydrogen and 89 oxygen by mass. Regardless
  of its source
• He observed many other compounds and observed
  that the elements that composed the compounds
  were always in a certain proportion by mass.
  This is referred to as law of definite
  proportions.

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John Dalton
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Daltons Atomic Theory

• John Dalton (1766-1844), an English schoolteacher
  and chemist
• Studied the experiments of Lavoisier, Proust, and
  many others.
• Based on this he developed his atomic theory.

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Daltons Atomic Theory

1. All matter is made up of atoms.
2. Atoms are indestructible and cannot be divided
   into smaller particles(Atoms are indivisible)
3. All atoms of one element are exactly alike, but
   they are different from atoms of other elements.

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Daltons Atomic Theory

• This gave chemist a model of the particle nature
  of matter, but it also raised a lot of questions.
• If all elements are made up of atoms then why are
  there so many different types of elements?
• What makes one atom different from another?

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Daltons Atomic Theory

• Experiments performed late in the 19th century
  began to explain the properties and behavior of
  substances
• This was done by the discovery of three smaller
  particles.
• Protons, electrons and neutrons

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Atomic Theory, Conservation of Matter and
Recycling

• What happens to the stuff you throw away? What
  happens to the atoms?
• As you have already learned matter is neither
  created nor destroyed, so what happens.

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What happens?

• When waste is incinerated or buried the atoms may
  combine with oxygen and other substances to form
  new compounds.
• The atoms are recycled

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Recycling

• Recycling has become a part of life now.
• Much of what you buy is either recycled or can be
  recycled.
• We have found the advantage of recycling
  materials and therefore atoms. We mimic what
  nature does and conserve our natural resources.

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Hypotheses

• Hypothesis testable prediction to explain
  observations.
• Hypotheses are based on observations.
• They can be proven correct or incorrect by the
  experiments that are designed to test them.

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Hypothesis
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Theories

• Theory- an explanation based on many observations
  and supported by the results of many experiments.
• As scientists gather new information, a theory
  may be revised or replaced.

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Theory
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Laws

• Scientific Law a fact of nature that is
  observed so often that it is accepted as the
  truth.
• Examples
• Sun rises in the east
• A law can generally be used to make predictions
  but does not explain why something happens.
• Theories explain laws.
• One part of Daltons atomic theory explains why
  the law of conservation of matter is true.

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Review

• What is the difference between a theory and a
  hypothesis?
• What is the Law of Conservation of Mass?

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The Discovery of Atomic Structure

• Daltons atomic theory was almost true.
• He assumed that atoms are the ultimate particles
  of matter and cant be broken up into smaller
  particles and that all atoms of the same element
  are identical.
• His theory needed to be modified with the
  discovery of electrons, protons and neutrons.

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The Electrons

• In 1897, JJ Thomson a British physicist
  discovered that the solid ball of model was not
  accurate.
• His experiments involved the use of a cathode
  ray.

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Cathode Ray

• Composed of a vacuum tube
• At the end of the tube is a piece of metal called
  on electrode which is connected to a metal
  terminal on the outside of the tube.
• The electrode becomes electrically charged when
  connected to a high voltage.
• When the electrode is charged rays travel in the
  tube from the negative electrode(the cathode) to
  the positive electrode (the anode)

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Cathode Ray
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Cathode Ray
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Thomsons Discovery

• What he found that the ray would bend towards a
  positive charged plate and away from a negatively
  charged plate.
• He concluded that the rays are composed of
  invisible negatively charged particles he called
  electrons
• Electron negatively-charged particle

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Early Atomic Model

• The early atomic model was referred to as the
  plum pudding model you could more closely
  relate it to a chocolate chip cookie
• Scientists believed that atoms were balls of
  positive charge with the negatively charged
  particles embedded in them.

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Changes in the Atomic Model
Plum Pudding Model
Solid Ball Model
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Ernest Rutherford

• In 1909 he carried out the first of the
  experiments that would reveal an arrangement far
  different from the plum pudding model

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Rutherfords Experiment

• Gold Foil Experiment
• He set up a lead-shielded box containing
  polonium, which emitted a positively charged beam
  of alpha particles.
• When the beam struck a sheet of gold foil, most
  of the particles passed straight through the foil
• However, some of the particles from the beam were
  deflected. Some were only slightly deflected and
  some bounced straight back.

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Gold Foil Experiment
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Gold Foil Experiment
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The Nuclear Model of the Atom

• Based on Rutherfords work the team devised a new
  model for the atom.
• Because some of the particles bounced straight
  back they concluded that atoms must have a dense
  central core called the nucleus
• Nucleus a small, dense, positively charged
  central core of an atom

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Nuclear Model
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Changes in the Atomic Model
Plum Pudding Model
Nuclear Model
Solid Ball Model
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Atomic Model

• It was hard for people to grasp that atoms
  contained a lot of empty space.
• When they looked at a rock, it was very difficult
  to see how most of this object could be empty
  space.
• If you were to enlarge an atom of hydrogen so the
  nucleus was the size of a golf ball, the electron
  would be a mile a way.
• In one drop of water there is 6,500,000,000,000,00
  0,000,000 (6.5 sextillion) atoms

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Atomic Numbers and Masses

• The nucleus of an atom is composed of protons and
  neutrons. Electrons move in the space around the
  nucleus.
• Atomic Number the number of protons in the
  nucleus of an atom.
• Every element has a unique atomic number.
  Therefore, it is the number of protons that
  determines the identity of an element.

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Atoms

• Protons - positively charged subatomic
  particles.
• Neutrons subatomic particle that does not have
  a charge, it is neutral
• Atoms have no overall charge.
• Therefore the number of protons and electrons
  must be equal.
• If you elements atomic number is 2
• 2 protons
• 2 electrons

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Mass Number

• Mass number - the number of protons and neutrons
  in the nucleus of an atom.
• Isotope atom of an element with a different
  number of neutrons and therefore a different mass
  number
• Ne-20 neon 20(10 protons, 10 neutrons)
• Ne-21 neon 21(10 protons, 11 neutrons)

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Neon
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Composition of Atoms

• You can determine the composition of any element
  if you know the
• Atomic number
• Mass number

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Atomic Mass

• Since elements have different isotopes the atomic
  mass is the weighted average.
• Located under the symbol on the periodic table.
• You can use the atomic mass to determine the mass
  number of a element
• You take the atomic mass rounded up

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Determining Atomic Mass
62.930 X 0.6917 43.529 64.928 X 0.3083 20.017
43.529 20.017 63.546
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Review

• What information does the atomic number give us?
• How do we determine the number of neutrons of any
  element?
• Who discovered the electron?
• What discovery did the Gold Foil Experiment led
  to?