Chapter 2 Atoms and Elements

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Chapter 2Atoms and Elements
2
Daltons Atomic Theory

• Elements are made of tiny particles called atoms.
• Each element is characterized by the mass of its
  atoms. Atoms of the same element have the same
  mass, but atoms of different elements have
  different masses.
• Chemical reactions only rearrange the way atoms
  are combined the atoms themselves are unchanged.

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Conservation of Mass

• Law of Mass Conservation Mass is neither created
  nor destroyed in chemical reactions.

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Daltons Atomic Theory

• Law of Definite Proportions Different samples of
  a pure chemical substance always contain the same
  proportion of elements by mass.
• Any sample of H2O contains 2 hydrogen atoms for
  every oxygen atom

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Daltons Atomic Theory

• Law of Multiple Proportions When two elements
  form two different compounds, the mass ratios are
  related by small whole numbers.
• It is possible for the same two elements to
  combine in different mass ratios to give
  different substances.
• Example NO and NO2

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The Discovery of Electrons

• Cathode-Ray Tube (Thomson, 18561940)
• Cathode rays
• consist of tiny
• negatively
• charged
• particles,
• now called
• electrons.

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The Discovery of Electrons

• Oil Drop Experiment (Millikan, 18681953)
  Applied a voltage to oppose the downward fall of
  charged drops and suspend them.
• Voltage on plates place 1.602176 x 10-19 C of
  charge on each oil drop.
• Millikan calculated the electrons mass as
  9.109382 x 10-28 grams.

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The Structure of Atoms

• Discovery of Nucleus Rutherford irradiated gold
  foil with a beam of alpha particles to search
  for positive charged particles. Most of the
  particles passed through but some were deflected
  at large angles, why?

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The Discovery of Electrons

• Thomsons conclusions
• The cathode rays are made of tiny negatively
  charge particles
• Every material tested contained these same
  particles
• Thomson believed that these particles were the
  ultimate building blocks of matter
• These particle became known as electrons

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The Structure of the Atom

• Thomson
• The structure of the atom is like plum pudding
• Negatively charged particle is a sphere of
  positive charge

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The Structure of the Atom

• The Plum Pudding model
• The mass of the atom is due to the mass of the
  electrons within
• The atom is mostly empty space because negative
  particles repel

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Radioactivity

• Curie and Becquerel discovered that certain
  elements would constantly emit small, energetic
  particles and rays
• These energetic particles could penetrate matter

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Radioactivity

• Rutherford discovered there were three type of
  radioactive emissions
• Alpha
• mass of 4x a hydrogen atom and charge
• Beta
• mass of 1/2000th of a H atom and - charge
• Gamma
• Energy rays not particles

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Rutherfords Experiment

• Shoot alpha particles at a very thin sheet of
  matter and show that they all pass through

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Rutherfords Experiment

• Results
• 98 of the particles went straight through
• About 2 of the particles went through but were
  deflected by large angles
• About 0.01 of the particles bounced off the gold
  foil

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Rutherfords Experiment
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Rutherfords Experiment

• Conclusions
• The atom contains a tiny dense center called a
  nucleus
• The atom is mostly empty space
• The electron are dispersed in the empty space
  around the nucleus
• The nucleus is positively charged
• The nucleus of the atom must have a particle of
  the same amount of charge but opposite in sign

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The Structure of Atoms
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The Structure of Atoms

• Atomic number ( Z) Number of protons in atoms
  nucleus number of electrons around atoms
  nucleus.

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The Structure of Atoms

• Mass Number (A)Number of protons (Z) Number of
  neutrons (N)

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The Periodic Table of the Elements
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Protons, Electrons and Neutrons

• Protons determines the element
• Number of protons atomic number
• Electrons determines the reactivity
• If neutral of electrons of protons
• If charged the sum of the charged particel must
  equal the charge
• Neutrons determines the nuclear stability
• The atomic mass (whole number) - the of protons
  the of neutrons

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Protons, Electrons and Neutrons

• Ions different number of electrons than neutral
• Cations have a positive charge, less electrons
  than neutral
• Anions have a negative charge, more electrons
  than neutral
• Isotopes different number of neutrons
• Can be stable or radioactive
• Have different atomic mass
• The elemental mass on the periodic table is
  usually the most common isotope
• The mass on the periodic chart is calculated from
  the sum of all the weighted masses of the
  naturally occurring isotopes

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Examples P, E, and N
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Example calculation of atomic mass

• If copper is 69.17 Cu-63 with a mass of
  62.9396amu and the rest Cu-65 with a mass of
  64.9278amu, what is coppers atomic mass?

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Patterns
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Patterns

• Metals shiny, ductile, malleable, conduct heat,
  conduct electricity
• Nonmetals dull, brittle,poor conductor of heat
  and electricity
• Metalloids semimetals, semiconductors, show
  properties of both metals and nonmetals

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Some Important Families

• Noble gases
• nonreactive

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Some Important Families

• Alkali metals
• Very reactive metals

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Some Important Families

• Alkaline Earth metals
• Reactive metals

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Some Important Families

• Halogens
• Reactive nonmetals

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Patterns and Ions
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Counting Atoms by the Mole

• A mole is defined as a unit having
  6.022x1023particles
• A mole can be used to determine the number of
  atoms or molecules in a sample.

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Counting Atoms by the Mole

• Recall To find the number of moles in a sample
  divide the number of grams by the molar mass
• To find the number of particle in that sample
  multiply the number of moles by the Avogadros
  number

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Example Counting Atom

• Determine the number of copper atoms in a penny
  weighing 3.10g.