Chapter 10 Introduction to Atom

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Chapter 10- Introduction to Atom
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• Element? substance that cannot be broken down
  into simpler substances. It is a matter made of
  atoms of only one kind
• ?119 Elements on Periodic Table
• ?ExampleIron is only made of iron atoms

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• Atom? smallest part of an element
• ?Consist of a nucleus surrounded by one or more
  electrons

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• Scientists who contributed to the knowledge of
  the Atom
• ? The Theory of the Atom has changed over time

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Democritus 440 BC

• A Greek Philosopher, who theorized that if you
  cut an object in half repeatedly, you would end
  up with a particle that could not be cut.
  (Atom)
• Atom (Greek word Atomas) meaning not able to be
  divided

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Aristotle (400BC)

• Greek Philosopher, who believed that you would
  never end up with a particle that could not be
  cut
• More Influential
• He was WRONG

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John Dalton (1700s)

• British School Teacher / Chemist
• Combined his idea of Element with Greek Theory
  of the Atom
• He proposed the following ideas about Matter

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• Matter is made up of Atoms.
• Atoms cannot be divided into smaller pieces.
• All the Atoms of an Element are exactly alike.
• Different Elements are made of different kinds of
  Atoms.

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William Crooke (1861)

• Start of Scientific Evidence
• (No Lab Experiments before this time)
• Glass tube that had almost all the air removed
  from it.
• The glass tube had two pieces of metal called
  Electrodes.

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• Electrode? is a piece of metal that can conduct
  electricity.
• Anode? an electrode that has a positive end.
• Cathode? an electrode that has a negative end.

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• ?Crooke hypothesized that the green glow in the
  tube was caused by rays, or streams of particles.
• ?Rays were called Cathode Rays
• ?Used in TV and Computer Screens

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J. J. Thompson (1897)

• Tested Crookes Experiment
• ?Was the Greenish Glow Light, or was it a stream
  of charged particles?
• ?Placed a Magnet beside the tube from Crookes
  experiment.

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• The beam is bent in the direction of the magnet.
• Light cannot be bent by a Magnet, as the beam
  couldnt be light.
• Conclusionmust be a charged particle that came
  from the cathode.

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• He showed that there was a mistake with Daltons
  Theory. The Atom could be divided into smaller
  pieces.
• Electrons ? negatively charged particle that
  exists in an Electron formation around an Atoms
  nucleus

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Ernest Rutherford (1909)

• Former student of Thompsons
• Designed an experiment to study the parts of the
  Atom
• He aimed a beam of small, positively particles at
  a thin sheet of Gold Foil
• He placed a coated screen around the foil
• The coating glowed when hit by the positively
  charged particles

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• ?Hypothesized that almost all the mass of the
  atom and all of its positive charge are crammed
  in the center of the atom called the Nucleus.
• Proton? positive charge particle present in the
  nucleus of all atoms.

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• ? An Atoms Electron have almost no mass.
• Neutron? neutral particle that has the same mass
  as a proton and is found in an atoms nucleus

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Niels Bohr (1913)

• Danish scientist who worked with Rutherford
• Studied the way Atoms react with light
• Electrons travel in a region surrounding the
  nucleus called the Electron Cloud.

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Parts of the Atom

• Size of an Atom
• Dot on your paper 2 Million Atoms
• Penny 20,000,000,000,000,000,000,000
• Atoms or 2 X 10²² Atoms of Copper and Zinc

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Nucleus (Center Atom)

• Proton ? positively charged particle
• --All Protons are identical
• Neutron ? no electrical charged particle
• --All Neutrons are identical
• Neutrons are slightly larger than Protons
• Majority of Mass in Atom

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Outside the Nucleus

• Electron ? negatively charged particle
• Has hardly any Mass
• Very Small (1,800 Electrons equal Mass of 1
  Proton)
• ? Electrons are found around the Nucleus in what
  is called Electron Clouds

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• Mass Proton Mass Neutron
• Mass Proton gt Mass Electron
• Mass Neutron gt Mass Electron

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How do Atoms in an Element Differ?

• Elements differ by their number of Protons,
  Neutrons, and Electrons in each Atom.
• ? The different numbers of each will give the
  Atom special properties

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Neutral Charge

• Elements can differ in charge
• ? The Element will either have a neutral charge,
  positive charge, or negative charge

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Atomic Number

• ? Is the number of protons in the nucleus of an
  atom of that element.
• ?The Atoms of different elements contain
  different numbers of protons.

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• Examples? Hydrogen has 1 proton, Carbon has 6
  protons, Oxygen has 8 protons, Iodine has 53
  protons
• ?Atoms of an element are identified by the number
  of protons because this number never changes
  without changing the identity of the element.

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Mass Number

• ? the sum of Neutrons and Protons in the nucleus
  of an Atom.
• ?To calculate the number of Neutrons in an
  isotope you must subtract the Atomic Number from
  the Mass Number of the isotope.

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Neutrons

• ?The atomic number is the number of protons, but
  what about the number of Neutrons in an Atoms
  nucleus.
• ?A particular type of Atom can have a varying
  number of neutrons in its nucleus

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Isotopes

• ? are Atoms of the same element that have
  different numbers of Neutrons.
• Carbon-12 (Normal)
• Carbon-13 (Isotope)
• Carbon-14 (Isotope)

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Radioactive Decay

• ?Many atomic nuclei are stable when they have
  about the same number of Protons and Neutrons.
• ?Some become unstable when too many or too few
  Neutrons.

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• Examples? Uranium and Plutonium
• ?Repulsion builds up because the nucleus must
  release a particle to become stable
• ?When particles are released, energy is given off

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• Radioactive Decay? release of nuclear particles
  and energy from unstable nuclei
• Transmutation? the changing of one element into
  another through radioactive decay
• Example Smoke Alarm

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Half-Life

• Half-Life? time needed for one-half of the mass
  of a sample of a radioactive isotope to decay.

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• Half Life Problem
• Tritium has a half-life of 12.5 years. If you
  start with 20 grams of Tritium, how much is left
  after 50 years?

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Carbon Dating

• ?Radioactive decay is useful in determining the
  age of artifacts and fossils.
• ?Carbon-14 Half-Life of 5,730 years.
• ?In a living organism, the amount of Carbon-14
  remains in constant balance with the levels of
  the isotope in the atmosphere or ocean.

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• ?The balance occurs because living organisms take
  in and release Carbon.
• ?While life processes go on, any Carbon-14
  nucleus that decays is replaced by another from
  the environment.
• ?When the plant and animal dies, the decaying
  nuclei no longer can be replaced.

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Calculating the Mass of an Element

• Atomic Mass ? of an Element is the weighted
  average of the Masses of all the naturally
  occurring Isotopes of that Element

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Atomic Mass Problem

• ? Chlorine-35 makes up 76 of all the Chlorine
  in nature, and Chlorine-37 makes up the other
  24. What is the Atomic Mass of Chlorine?

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Forces of an Atom

• Gravitational Force depends on mass and
  distance apart
• -- since atom very small this force is small
• Electromagnetic Force objects that have the
  same charge repel each other, while objects with
  opposite charge attract each other
• -- strong force

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• Strong Force -- Protons push away from each
  other
• -- strongest force
• 4. Weak Force Occurs in Radioactive Atoms when
  Neutrons change to Protons
• -- weak force