The Structure of the Atom

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The Structure of the Atom

• History, Structure, Properties and Forces

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I. Early Theories of Matter

• before the early 1800s many Greek
  philosophers thought that matter was formed of
  air, earth, fire and water

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II. Democratus

• 1) first to propose atomos - matters as small
  indivisible particles
• 2) said they move through empty space
• 3) different properties of matter due to changes
  in arrangement of atoms

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III. Daltons Postulates

• 1) Small- elements are made of atoms
• 2) Identical -atoms of an element are identical
  in their masses - different elements have
  different masses

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Dalton a English School Teacher

• 3) Created - matter is neither created or
  destroyed - chemical reactions - rearrange atoms
• 4) Compounds - atoms only combine in small,
  whole number ratios such as 11, 12, 23...

• In truth, Dalton never ruled out the possibility
  of subatomic structure. He just knew that the
  state of the art in the early 1800's did not
  allow the physical structure of an atom to be
  probed.

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Postulate 4 is actually the Law of Definite
Proportions, by Joseph Louis Proust in 1797.

• a) a given chemical compound always contains the
  same proportion by mass of its constituent
  elements or...
• b) the relative amount of each element in a
  particular compound is always the same,
  regardless of preparation or source.

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IV. Subatomic Particles and the Nuclear Atom

• 1) J.J. Thomson - a) discovered electron using
  Cathode ray tube b) was able to determine
  charge to mass ratio but was unable to calculate
  exact charge
• 2) Robert Millikan - determined the charge of the
  electron using an oil drop experiment - was very
  close to the accepted value we have today

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• 3) models of atoms - J.J. Thomson purposed a
  plum pudding model in which a evenly distributed
  positive circle was imbedded with electrons
  throughout.

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4) Rutherford, Geiger, Marsdens propose a
different model -Gold Foil Experiment
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• a) Ernest Rutherford a ex student of Thompson had
  been studying alpha particles since 1898. In
  fact, he discovered them. In 1909 he was
  confronted with some rather bizarre
  alpha-particle behavior that he had to explain.
  What was the behavior, exactly?
• b) Hans Geiger and Ernest Marsden aimed a stream
  of alpha particles at a thin gold foil for
  several months in 1909.

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• Rutherfords associates discovered
• c) almost all of the alpha particles went through
  the gold foil as if it were not even there..
• d) some of the alpha particles were deflected
  only slightly, usually 2 or less.
• e) a very, very few (1 in 8000 for platinum foil)
  alpha particles were turned through an angle of
  90 or more. (Rutherford cites 1 in 20,000 for
  gold in his 1911 paper.)

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• f) from these results it was concluded
• that a extremely small dense area of
  positive charge existed (area that reflected
  alpha particles straight back)
• that most of the atom was empty space (most
  particles passed straight through)
• electrons have very minimal mass (particles
  that were only slightly reflected by the alpha
  particles)

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5) The Modern Atom

• A) about 1920 - eight years after the Gold foil
  experiment - Rutherford - called positive charged
  area a nucleus with a positive proton.
• B) 1932 - James Chadwick discovered the Neutron

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Properties of Subatomic Particles
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V. Comparing Atoms

• 1) atomic number - the number of protons in
  an atom also equals the electrons in a neutral
  atom APE
• all atoms of an elements have the same number
  of protons
• 2) elements - all have the same type of atoms
  (same number of protons) but can have different
  numbers of neutrons and electrons

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• 3) isotopes - atoms of an element that have a
  different number of neutrons are isotopes of that
  element
• a) mass number - is the number of protons
  the number of neutrons
• b) nuclide symbol - is a symbol that notes the
  number of protons and neutrons of a specific atom
  - Carbon -14
• mass 14
  notes
  
• C
  6 protons
• atomic 6 8
  neutrons

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Atomic Mass

• Weighted average mass of all the naturally
  occurring isotopes of an element
• Calculated by the sum of all the isotopes times
  their mass in amus

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4) ions - are atoms of an element that have
different numbers of electrons

• a) cations - have fewer numbers of electrons than
  the neutral atom and carry a positive charge.
  (metals form cations)
• Mg 12 Mg2 12
• 12-
  10-
• 0
  2
• magnesium loses 2 electrons to form a positive
  2 cation as do all members of the alkaline earth
  metals

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• b) anions - atoms of an element that have more
  electrons than their neutral atom (non-metals
  form anions)
• Br 35 Br-35
• 35- 36-
• 0 -1
• bromine gains one electron to become a
  negative one charged anion as do all halogens.

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VI. Nuclear Chemistry

• 1) protons and neutrons are in the nucleus
• and compose almost all the mass of the atom
• 2) they are tightly packed in the nucleus
  -neutrons have no charge and protons have a
  positive charge
• 3) why dont the positively charged protons repel
  each other - why does the nucleus stay together?

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• 4) the strong force holds the protons to the
  neutrons and the protons to the protons in the
  nucleus
• a) one of the 4 basic forces and is about 100
  times stronger than the electric force
• b) is a short range force that quickly
  becomes weak as protons and neutrons get further
  apart
• c) the electric force is a long range force,
  so protons that are far apart repel each other

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• d) the larger the atom the weaker the strong
  force is --gt larger atoms tend to decay (nuclear
  decay)
• e) also, atoms with certain numbers of protons
  tend to decay--if they have the same number of
  protons and neutrons they tend to be more
  stable--different numbers--more unstable

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• 5) 4 forces in nature
• a) strong force
• b) weak force
• c) electrical force
• d) gravitational force
• 6) Usages of Nuclear Chemistry
• a) production of power
• b) killing bacteria in food products
• c) medicine -cancer treatments along with
  other usages

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• 7) Radiation-Radioactivity
• a) radiation is electrically charged particles
  or waves emitted by an energy source or decaying
  atoms.
• b) radioactivity - is radiation from an
  unstable atom that is splitting or undergoing
  decay. There are three types of radioactive
  materials
• Alpha radiation - harmful if inhaled or
  otherwise enter the body - can be stopped by
  clothes, skin or a sheet of paper.

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• Some producers of alpha particles are among
  the longest lasting waves
• beta particles - more penetrating power than
  alpha but most serious when inhaled or ingested
  -- tend to concentrate in certain body parts,
  such as bone -can cause serious health problems
  with minimal exposure
• gamma radiation-- highest energy levels
  -similar to x-rays, can penetrate the body and
  cause direct damage to internal organs

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Types of Radiation

• 1. Alpha Radiation an alpha particle emitted
  from radioactive nuclei, consists of 2 protons
  and 2 neutrons, but no electrons
• Not very harmful
• Large atoms are not very stable and need to
  decrease mass
• What is the charge and mass of an alpha particle?
• 2 and 4amu
• Example
• 22688Ra ? 22286Rn 42He
• Radium Radon Alpha Particle

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Types of Radiation

• Beta Radiation fast moving electron emitted from
  a radioactive element called a Beta Particle.
• Can cause serious health problems especially in
  bones
• Atoms want to have a 1 1 neutron to proton
  ratio
• Beta emission is used to decrease the neutron to
  proton ratio.
• What is the charge and mass on a beta particle?
• -1 and 0
• 146C ? 147N 0-1ß

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Types of Radiation

• Gamma Radiation Gamma rays are released from
  radioactive nuclei.
• Gamma rays have no mass or charge.
• Gamma rays are very harmful and have a very high
  energy

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• Effects of radioactive particles on biological
  systems --can alter cellular function
  particularly DNA--carries the cells genetic
  code-causing birth defects-can create mutated
  forms of cells that can cause cancerous growths
• 8) Fusion-Fission
• a) fusion uniting of nuclei of two light
  elements to form heavier nucleus- example sun--

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• b) fission - a heavy nuclide splits into two or
  more intermediate- sized fragments when hit in a
  particular way by a neutron --utilized by nuclear
  power plants --to make nuclear bombs