the history of the atom!

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Atomic StructureHistorical look at the Atom
440 BC
Democritus
1803
John Dalton
1897
Joseph John (J.J.) Thomson
1911
Ernest Rutherford
1922
Niels Bohr
1932
James Chadwick
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Democritus

• DISCONTINUOUS THEORY OF MATTER
• All matter is composed of atoms, which are bits
  of matter too small to be seen. These atoms
  CANNOT be further split into smaller portions.
• Aristotle ? Continuous theory of Matter

Where does the term atom come from? In Greek,
the prefix "a" means "not" and the word "tomos"
means cut. Our word atom therefore comes from
atomos, a Greek word meaning uncuttable.
Democritus
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John Dalton (1803)

• proposed the first theory of the nature of
  matter in stating that all matter was composed of
  atoms.

• Dalton based his theory on three scientific
  principles

• the Law of Conservation of Mass (Lavoisier)
• the Law of Definite Proportions (or Law of
  Constant Composition) (Proust)
• the Law of Multiple Proportion (his own)

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Law of Conservation of Mass

• Established in the 1760s by Antoine Lavoisier
• in any chemical reaction, mass is neither created
  nor destroyed (reactants will have the same mass
  as products in a chemical change)
• Ex. C6H12O6 O2 ? CO2 H2O
• 180 grams 70 grams
  110 grams 140 grams
• Ex. CO2 H2O ? C6H12O6
  O2
• 110 grams 140 grams
  180 grams 70 grams

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Law of Definite Proportions

• Established in 1799 by the French Chemist Joseph
  Louis Proust
• States that in a pure compound, the elements are
  always present in the same definite proportion by
  mass.
• Ex. H2O
• 2.0 g of hydrogen will combine with 16g of oxygen
  to produce 18 g of H2O
• and
• 4 g of hydrogen will combine with 32 g of oxygen
  to produce 36 g of H2O
• Ratio of HO is always 216 or 18 (by mass)

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Law of Multiple Proportions

• When two or more compounds are formed from the
  same pair of elements, the masses of one element
  combines with a fixed mass of the other element
  to form simple whole-number ratios of those
  elements

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Example Two different compounds are formed by
the elements carbon and oxygen. The first
compound contains 42.9 by mass carbon and 57.1
by mass oxygen. The second compound contains
27.3 by mass carbon and 72.7 by mass oxygen.
57.1 g O / 42.9 g C 1.33 g O per g
C 72.7 g O / 27.3 g C 2.66 g O per g
C Dividing the mass O per g C of the second
(larger value) compound 2.66 / 1.33 2
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John Dalton cont.

• Four basic ideas in Daltons chemical atomic
  theory (Postulates of Atomic Theory)
• Based upon 3 Laws
• Matter is composed of tiny indivisible particles
• all atoms of the same element are identical in
  mass, but differ from atoms of other elements
• atoms only combine in definite fixed numerical
  ratios such as 11, 12, 21
• chemical change consists of a reshuffling of
  atoms, the individual atoms themselves remain
  intact

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The 1st Subatomic Particle General Info
Electrode gt substance that allows electrons to
enter or exit Anode gt positively charged
electrode Cathode gt negatively charged
electrode
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William Crookes

• Determined the flow of current was in a straight
  path
• The flow always originated from the cathode
  (negative electrode) and flowed to the anode
  (positive electrode)
• Called these rays cathode rays

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J.J. Thomson

• 1897, used magnetic and electric fields to alter
  the cathodes path which showed the rays must be
  negatively charged
• Determined the charge to mass ratio of this
  negative particle (e-/m)
• Also felt there was a positive balance to this
  negative charge

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positive charged matter
negative charged particle
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Robert Millikan

• American Physicist
• 1909 determined the charge of an electron and
  the mass of an electron
• Oil drop experiment
• Used Thomsons charge to mass ratio (e-/m)
• 1.602 x 10-19 coulombs (charge of electron)
• 9.109 x 10-28 g (mass of electron)
• (100 years later within 1 of the known value)

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• 1909 conducted the alpha scattering experiment
  (better known as the gold foil exp.)
• used a beam of high speed alpha particles to
  study the interaction with metal foils (gold
  mostly)
• found that over 99 of the alpha particles went
  through the foil
• determined the atom was mostly empty space with a
  positively charge core (nucleus)

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• Deflections were a result of electrical repulsion
• Determined nearly all the mass of an atom is
  located in the nucleus (little nut)
• This positive charged core only occupied a small
  portion of the atoms volume

"Planetary Model"
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DUAL NATURE OF LIGHTCharacteristics of energy as
waves

• Wavelength symbol ? lambda
• Distance between any point on a wave to a
  corresponding point on the next wave
• Expressed in meters (m) as the wave shortens ?
  nm (10-9)
• Amplitude
• Represents the height of the crest (or depth of
  a trough)
• Represents the intensity of the radiation (ie.
  visible light)
• Frequency symbol ? nu
• Number of cycles a wave passes a given point
  every second
• Expressed as.. 1 or waves or sec-1
  or Hertz
• sec sec
• Speed symbol ? c
• Represents the speed of light in a vacuum
• 3.00 x 108 m
• sec

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EM Spectrum

• Radio, Microwave, Infrared, Visible light,
  Ultraviolet, X-Rays, Gamma rays
• Emission spectra
• represents energy released by an electron as it
  returns to ground state from excited state
• Absorption spectra
• represents energy absorbed by the electron as an
  electron moves to an excited state

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Formulas you need

• l c and E h x u
• u
• Wavelength speed of light Energy
  Plancks constant x freq.
• frequency

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Terminology

• Ground state lowest energy level for an
  electron when an atom is in its most stable
  energy state
• Excited state a higher energy state of an
  electron (energy is absorbed by electron)
• Electromagnetic spectrum range of energy that
  exhibits wave-like behavior as it travels through
  space.
• Consists of both electric and magnetic field
  components.
• classified according to the frequency of its
  wave.

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Bohr ModelQuestion Why didnt the atom collapse
on itself?
ANSWER

• electrons move within defined orbits
• electrons have a fixed energy within the orbit
  and do not radiate energy as they move (DUE TO
  UNDERSTANDING OF THE ENERGY ASSOCIATED TO THE
  ELECTROMAGNETIC SPECTRUM)
• electrons can move from orbit to orbit, but can
  not be between orbits
• the orbits are considered energy levels
• the Bohr model was a precursor to the quantum
  mechanical model

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Bohr Model
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Parts of Atom

• Electrons
• negatively charged subatomic particle
• mass 9.11 x 10 -28 g
• Protons
• positively charge subatomic particle
• mass 1.67 x 10 -24 g
• Neutron Discovered by James Chadwick ( 1932)
• Subatomic particle with no charge
• mass 1.67 x 10 -24 g

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Terminology

• Atomic number
• number of protons in the nucleus
• due to atoms electrical neutrality, also
  indicates the number of electrons
• on the periodic table above or below the symbol
  of the atom

• Isotope
• atoms with the same number of protons but with
  different number of neutrons

• Atomic mass
• weighted average of all the isotopes of a
  particular element
• on the periodic table above or below the symbol
  of the atom

• Mass number
• number of neutrons and protons in the nucleus
• mass number - atomic number neutrons