Basic Structure of the Atom

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Basic Structure of the Atom

• Evidence Supporting the Atomic Theory

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Democritus

• Ancient Greek
• Matter made up of atomos
• Atoms can not be
• Created
• Destroyed
• Further Divided

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Antoine Lavoisier (Late 1700s)

• Founder of Modern Chemistry
• Explained O2 role in combustion
• Concept of element as basic substance
• Conservation of matter theory
• Measured the mass of substances before and after
  a chemical reaction
• Performed carefully controlled experiments in
  closed systems
• Beheaded during French Revolution

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William Proust (1799)

• Devised the law of constant composition
• A given compound always contains the same
  elements in the same proportions

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Dalton

• Proposed Atomic Theory in 1803
• Half a Century until well accepted
• 100 years until proven
• Each element
• Composed of indivisible and indestructible atoms
• Atoms of different elements are different
• atoms of the same element are the same
• Atoms of different elements combine to form
  compound atoms (molecules)

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Michael Faraday (1839)

• Developed fundamental theories of electricity,
  magnetism, and light
• Theorized the structure of atoms are
• somehow related to electricity
• 1832 - Electrochemistry

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J. J. Thomson

• Discovered electrons in 1897
• Plum Pudding Model Positive and negative
  charges scattered randomly throughout atom
• Instantly accepted

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Experiment

• Passed a cathode ray from a cathode to an anode
  with a hole in it that allows a small amount of
  the cathode ray to pass through
• Cathode ray passes through a tube surrounded by
  both poles of a magnet and electrically charged
  plates
• Magnetic field turned on, the ray is deflected
  upwards
• Electric field is turned on, the ray is deflected
  downward
• Magnetic and electric fields are off, the ray
  travels straight

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Becquerl (1896)

• Discovered spontaneous emission of radiation from
  an element - radioactivity
• Accidently placed uranium salts on top of an
  unexposed photographic plate which was wrapped in
  paper and in a dark desk drawer
• Developed plate saw silhouette of salts
• Led Pierre and Marie Curie to discover radium and
  polonium by isolating them from pitchblende

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Millikan (1909)

• Measured charge and mass of the electron
• Experiment
• measured the effect of an electric field on the
  rate at which charged oil drops fell under the
  effect of gravity
• used x-rays near the droplet to charge them
• the oil droplets fall could be accelerated,
  retarded, or even reversed, depending on the
  charge on the droplet and the polarity voltage of
  the plate

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Rutherford

• Gold foil experiment 1909
• Mass concentrated in very small core at
• the atoms center (nucleus)
• Nucleus positive, negative electrons moving
  around it
• Contributions
• Field of Nuclear Physics (1898, alpha and beta
  particles)
• Radioactive decay
• Rectify Periodic Table

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Experiment

• Fired alpha particles at gold foil
• Most passed through the foil, a few were
  deflected
• When alpha particles (2) closely approaches the
  gold nucleus (79), it undergoes a strongly
  repulsive interaction

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Bohr

• Doctoral Thesis 1911 Theory of Electrons
• In 1913, discovered electrons revolve around the
  nucleus in energy levels (Einstein and Planck)
• Energy levels closest to nucleus have low energy
• Energy levels increase in energy with distance
  from the nucleus
• Electrons gain and lose energy by moving between
  energy levels (quantum)
• This is an enormous achievement Einstein

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Moseley (1914)

• Determined the atomic numbers of each element
• bombarded different elements with energetic
  electrons and studied the x-rays they emitted
• observed that the frequency of the x-rays were
  different for each element
• arranged the frequencies in order by assigning
  each element a unique, integral number, which he
  called the atomic number

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Modern Atomic Model

• The atom consists of three main particles
• Protons (positive)
• Neutrons (neutral)
• Electrons (negative)
• Two main parts
• Nucleus
• Electron cloud

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Atomic Model (cont.)

• Nucleus contains
• Protons ()
• Neutrons (0)
• Nucleus surrounded by
• electron cloud
• Negative charge due to electrons

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Atomic Structure
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Atomic Structure
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Atomic Mass Units

• Mass measured in atomic mass units
• For protons and neutrons
• 1 amu is defined as 1/12 the mass of a carbon
  atom containing 6 protons and 6 neutrons
• 1 amu is also the mass of 1 proton or 1 neutron
• An electron has a mass of 1/2000 amu

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Key Terms

• Atomic number the number of protons in the
  nucleus of an atom.
• Mass number the sum of the number of protons
  AND the number of neutrons in the nucleus.
• Mass protons neutrons

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Atomic Symbol and Mass Number
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Isotopes

• Isotopes are atoms of the same element with
  differing numbers of neutrons.
• Isotopes have different masses

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Isotopes
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Isotopes of Carbon
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How do you record the mass of a group of isotopes?

• Because most elements have more than one isotope,
  each element is given an average atomic mass
• The average atomic mass is the average mass of
  the mixtures of its isotopes

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How do you calculate the average atomic mass of
an atom?

• The number of naturally occurring isotopes, their
  masses, and their percent abundances must be
  known.
• Example Lithium has 2 isotopes Li-6 (mass
  6.015 amu and 7.5 abundance), and Li-7 (mass
  7.017 amu and 92.5 abundance). What is its
  average atomic mass?

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How do you calculate the average atomic mass of
an atom?

• Calculate the average atomic mass of silicon.
  The three silicon atoms have masses of 27.98 amu,
  28.98 amu, and 29.97 amu with relative abundances
  of 92.23, 4.67, and 3.10, respectively.

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What is radioactivity?

• Emission of high energy radiation or particles
  from the nucleus of a radioactive atom
• The atoms of radioactive elements are held
  together less securely than nonradioactive
  elements
• Particles of energy can escape from all nuclei
  with atomic numbers 84 or higher (radioactive
  decay)
• The nuclei of these elements are unstable
• In elements lt 20 Atomic Number, np 11
• In elements gt 20 Atomic Number, np 1.51
• The atoms nucleus is held together by the strong
  force
• prevents protons from pushing each other out of
  the nucleus
• only acts across very small distances

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How do you write the symbol for a nuclide?

• The symbol gives atomic , mass , and chemical
  symbol

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mass
K
chemical symbol
19
atomic
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What is nuclear radiation?

• Radiation given off by radioactive nuclides
• There are three types
• alpha particles (? particles)
• beta particles (? particles)
• gamma rays (? rays)
• Only gamma rays are a type of electromagnetic
  radiation!!

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What are alpha particles?

• Given off when a nucleus releases 2 neutrons and
  2 protons
• Same thing as a helium nucleus
• Has a charge of 2 and an atomic mass of 4
• Largest and slowest form of radiation
• Least penetrating can be stopped by a sheet of
  paper
• Used by smoke alarms (americium)

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What are beta particles?

• Neutrons can spontaneously decay into a proton
  and an electron
• The electron is the beta particle
• The proton can decay into a neutron and a positron

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What is a positron?

• A positron is similar to an electron, only with a
  positive charge
• Positrons are considered beta particles too
• Beta particles are much faster and more
  penetrating than an ? particle

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What are gamma waves?

• Most penetrating and potentially dangerous form
  of radiation
• Not made of particles
• Are electromagnetic waves with high frequency and
  energy
• Have no mass, no charge, and travel at the speed
  of light
• Usually released along with ? and ? particles
• thick blocks of lead and concrete are commonly
  used for barriers

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What is transmutation?

• Process of changing one element to another
  through nuclear decay
• Atomic mass of the decayed nuclide equals the
  sum of the mass of the newly formed nuclide and
  the emitted particle

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How do you determine the mass of the new nuclide?

• If the particle is an alpha particle, subtract
  the mass of the ejected particle from the mass of
  the old nuclide.

Alpha particle emission
Pb
He
214
218
Po

4
82
2
84
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How do I calculate the mass of the nuclide when
it loses a beta particle?

• Because a beta particle is the product of the
  decay of a neutron, a proton will be left behind
  when the e is ejected.

e
214
Bi

0
214
Pb
83
-1
82
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Charged Atoms

• In a neutral atom, the number of protons equals
  the number of electrons. The positive and
  negative charges balance out, leaving the atom
  with 0 net charge
• In a charged atom or ion, there is an uneven
  number of protons and electrons, so the atom will
  have either a positive or negative net charge

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Ions
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What is half life?

• Some nuclides of radioactive isotopes may require
  a long time to decay
• Half life is the amount of time it takes for half
  the nuclides in a sample of a given radioactive
  isotope to decay
• It can vary widely among the radioactive isotopes
• Can determine amount of a radioactive sample that
  will remain after a given amount of time with the
  half life

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Example

• Carbon 14
• At the beginning, there is 100. Its half life
  is 5730 years. So, after 5730 years, there will
  be only half, or 50, left. After another 5730
  years (11,460 total), there will be half of 50
  left, or 25. After another 5730 years (17,190
  total), there will be half of 25 left, or only
  12.5 of the original amount remaining.

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Is there a formula for half-life calculations?

• Amount remaining (initial amount)(1/2)n
• n number of half-lives that have passed
• n also can equal t/T, where t the elapsed time,
  T length of half-life
• Both t and T have to be in the same units

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What is carbon 14 dating?

• Radioactive materials - In your body
• Carbon 14 emits beta particles and decays into
  nitrogen
• Measuring carbon 14 to carbon 12 allows
  determination of approximate age of material

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How can you measure radioactivity?

• Cloud Chamber contains a gas cooled to a
  temperature below its condensation point
  droplets of the gas condense around the
  radioactive particles, which leave a trail that
  shows up along the chamber lining.
• Geiger counter produces an electric current in
  the presence of a radioactive substance.

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What is fission?

• Nuclear fission is the splitting of an atomic
  nucleus into two smaller nuclei
• Word fission means to divide
• Large nuclei with atomic numbers above 90 can
  undergo nuclear fission
• U 235, when bombarded by a neutron, splits to
  produce Ba 141, Kr 92, three neutrons and ENERGY!

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What is a nuclear chain reaction?

• Neutrons released from one fission reaction
  collide with another atom to cause another
  fission reaction.
• A continuous series of fission reactions is
  called a chain reaction.
• Huge quantities of energy are released with many
  simultaneous nuclear reactions.
• An uncontrolled chain reaction causes a nuclear
  explosion.

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Nuclear Chain Reaction
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Nuclear Reactor

• Nuclear reaction controlled with cadmium and
  boron control rods that absorb neutrons
• Generate heat (energy) from U-235 and heats the
  coolant water
• The hot coolant water then heats water that is
  used to drive steam-driven turbines, which
  produce electricity
• Problem Spent fuel rods are VERY hazardous
  waste and buried underground
• ½ life of U-235 is 713,000,000 years

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Schematic of a Nuclear Power Plant
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What happened at Chernobyl?
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What is Fusion?

• Fusion Bind together
• Joining of 2 less stable nuclei (lt60) into one
  stable nuclei
• Example Sun
• 4 1H 2 e --gt 4He 2 neutrinos 6 photons
• A temperature of 5,000,000 K required to overcome
  electrostatic repulsions between the nuclei

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Fusion Reaction

• In the picture to the
• right, two types of
• hydrogen atoms,
• deuterium and tritium,
• combine to make a helium
• atom and an extra particle called a neutron
• This process releases four times as much energy
  as the fission of a uranium nucleus

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Medical Uses for Radiation

• Treating Cancer Kills cancer and healthy cells
  as well
• Radiotracers emits non-ionizing radiation and
  is used to signal the presence of an element
• Used in studying blood flow patterns, uptake of
  thyroid gland, emptying rate of gallbladder
• Used in research experiments to trace amounts of
  chemicals in the system ( tertiary oil recovery)