Ch. 4 The Structure of the Atom

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Ch. 4 The Structure of the Atom
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Early Philosophers

• Democritus (460-370 BC)
• First to propose matter
• Is made of tiny particles, atomos
• Matter is composed of empty space in which atoms
  move
• Atoms are solid, indestructible, indivisible
• Ahead of his time

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Early Philosophers

• Aristotle (384-322 BC)
• Influential on the denial of atoms
• Did not believe in nothingness of empty space
  that existed
• Rejected Democritus ideas
• Aristotles denial of atoms went unchallenged for
  2,000 years

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John Dalton (1766-1844)

• Daltons Atomic Theory
• Matter composed of small particles called atoms
• Atoms of an element are identical
• Atoms cannot be created, divided or destroyed
• Different atoms combine in simple, whole number
  ratios in compounds
• In a chemical reaction, atoms are separated,
  combined or rearranged.

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Atoms

• Smallest particle of an element that retains the
  properties of the element
• World Population 6 000 000 000
• Atoms in Penny 29 000 000 000 000 000 000 000

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Discovery of the Electron

• In 1897, J.J. Thomson used a cathode ray tube to
  detect the presence of a negatively charged
  particle
• Cathode ray tubes pass electricity through a gas
  that is contained at a very low pressure

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Conclusion from the Study of the Electron

• Cathode rays have identical properties regardless
  of the element used to produce them. All
  elements contain identical charged particles.
• Atoms are neutral, so there must be a positive
  particle in the atom to balance it.
• Electrons have so little mass that atoms must
  contain other particles that account for most of
  the mass.

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Thomsons Atomic Model

• Thomson believed that electrons were like plums
  embedded in a positively charged pudding
  therefore, it was called the Plum Pudding Model

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Rutherfords Gold Foil Experiment

• Alpha particles were positively charged from
  helium nuclei
• Particles were fired at a thin sheet of gold foil
• Particle hits on the detecting screen (film) were
  recorded

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Rutherfords Findings

• Most particles passed through the foil
• Some particles were deflected
• VERY FEW were greatly deflected
• CONCLUSIONS
• There is a nucleus
• It is small
• It is tiny
• It is positively charged within the atom

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Atomic Particles
Particle Symbol Location Relative Electrical Charge Relative Mass Actual Mass (g)
Electron e- In the space surrounding the nucleus 1- 1/1840 9.11 X 10-28
Proton P In the nucleus 1 1 1.673 X 10-24
Neutron n0 In the nucleus 0 1 1.673 X 10-24
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Atomic Number (Z)

• Number of protons in the atom
• Since atoms are neutral, number of protons and
  number of electrons are the same
• Therefore,
• Atomic number protons electrons

Hydrogen 1 H 1.008
Chemical Name
Atomic Number
Chemical Symbol
Average Atomic Mass
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Mass Number

• Mass number is the sum of the protons and
  neutrons in an atom
• Mass number p n0

p n0 e- Mass
Sodium 11 12 11 23
Potassium 19 20 19 39
Bromine 35 45 35 80
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Isotopes

• Isotopes are atoms of the same element that have
  different mass numbers due to a different number
  of neutrons
• Same number of protons, but different number of
  neutrons
• neutrons mass number atomic number

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Isotope Protons Electrons Neutrons
Hydrogen 1 (protium) 1 1 0
Hydrogen 2 (deuterium) 1 1 1
Hydrogen 3 (tritium) 1 1 2
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Atomic Mass

• Atomic mass is the average of all the isotopes of
  that element
• Carbon 12.011

Isotope Symbol Composition of nucleus in nature
Carbon 12 12C 6 protons 6 neutrons 98.89
Carbon 13 13C 6 protons 7 neutrons 1.11
Carbon - 14 14C 6 protons 8 neutrons lt0.01
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Calculating Atomic MassExample on page 103

• Chlorine 37
• Atomic mass 36.966 amu
• abundance 24.230
• Mass contribution
• (36.966 amu)(24.230) 8.957 amu
• 26.496 amu 8.957 amu
• 35.453 amu

• Chlorine 35
• Atomic mass 34.969 amu
• abundance 75.770
• Mass contribution
• (34.969 amu)(75.770)
• 26.496 amu
• Atomic mass of Chlorine

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Nuclear Symbol

• 235
• 92

Mass Number (protonsneutrons)
U
Element Symbol
Atomic Number ( of protons)
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Unstable Nuclei and Radioactive Decay

• A chemical reaction involves the change of one or
  more substances into new substances
• The number of protons in the nucleus determines
  the identity of the atom
• During traditional chemical reactions, changes do
  not occur to the nucleus therefore, the identity
  of the atom does not change.
• In nuclear reactions, the atoms nucleus changes
  therefore, the identity of the atom changes

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Radioactivity

• In 1890s, scientists noticed that some substances
  emitted radiation in a process called
  radioactivity.
• The rays emitted are called radiation.
• Radioactive atoms emit radiation because the
  nuclei are unstable.

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Types of Radiation

• Alpha Radiation
• Made up of alpha particles
• Contains two protons and two neutrons
• Has a 2 charge
• Attracted to negatively charged plate
• Equivalent to a helium-4 nucleus
• Limited to VERY large nuclei

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2. Beta Radiation

• Consists of fast moving electrons called beta
  particles
• Each beta particle is an electron with a 1-
  charge
• Attracted to positive plate
• Converts a proton into a neutron

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3. Gamma Radiation

• Gamma rays are high energy radiation that possess
  no mass
• No electrical charge
• Accompany alpha and beta radiation
• Account for most of the energy lost in radiation

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