Pearson Prentice Hall Physical Science: Concepts in Action

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Pearson Prentice Hall Physical Science Concepts
in Action

• Chapter 4
• Atomic Structure

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4.1 Studying Atoms

• Objectives
• 1. Explain Daltons atomic theory
• 2. Explain the contributions of Thomson and
  Rutherford

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• Democritus (4th century B.C) said the universe
  was made of invisible units called atoms
• In 1808, an Englishman named John Dalton proposed
  a theory
• 1. every element is made of tiny unique particles
  called atoms that cannot be subdivided
• 2. atoms of the same element have the same mass
  atoms of different elements have different masses
• 3. compounds contain atoms of more than one
  element
• 4. in any particular compound, atoms of different
  elements always combine in the same way

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Thomson Rutherford

• Q What did Dalton notice that all compounds
  have in common?
• A Compounds contain atoms of more than one
  element
• JJ Thomson (1856-1940) used a sealed tube of gas
  to show that particles are charged

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• In Thomsons experiment, the particles repelled
  the negative plate and were attracted to the
  positive plate
• Q What was the charge on the particles in the
  beam of the gas?
• Thomsons experiments provided the first evidence
  that atoms are made of even smaller particles

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• Q How do objects of the same charge behave when
  they come close to one another?
• Thomson developed the plum pudding model of the
  atom
• His model suggested that negative charges were
  evenly scattered throughout an atom filled with a
  positively charged mass of matter

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• Ernest Rutherford (1871-1937) discovered fast
  moving particles in uranium that he called alpha
  particles
• Uranium is a radioactive substance
• He used a zinc sulfide screen, gold foil and
  alpha particles to produce a very famous
  experiment in which he showed that the positive
  charges are not evenly dispersed throughout the
  atom

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• Rutherford concluded that atoms must have a
  concentrated central core
• Definition a nucleus is a dense, positively
  charged mass located in the center of the atom
• According to Rutherfords model, all of an atoms
  positive charge is concentrated in its nucleus
• Today, much of an atom consists of empty space
  outside the positively charged nucleus

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4.2 The Structure of an Atom

• Objectives
• 1. Describe the three subatomic particles
• 2. Compare protons, electrons and neutrons in
  terms of physical properties
• 3. Explain how atoms of one element are different
  from atoms of other elements
• 4. Describe the difference between 2 isotopes of
  the same element

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Three Subatomic Particles

• Protons, neutrons electrons are subatomic
  particles
• Definition a proton is a positively charged
  subatomic particle that is found in the nucleus
  of an atom
• Definition an electron is a negatively charged
  subatomic particle found in the space OUTSIDE THE
  NUCLEUS (ORBITALS)
• Definition a neutron is a neutral subatomic
  particle found in the nucleus of an atom

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Comparing Protons, Neutrons Electrons

• Protons, neutrons and electrons can be
  distinguished by mass, charge location in an
  atom
• The mass of a neutron is almost exactly equal to
  the mass of a proton
• Electrons are nearly 2,000 times less massive
  than either a proton or a neutron

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Isotopes

• Definition isotopes are atoms of the same
  element that have different numbers of neutrons
  and different mass numbers
• Isotopes element has same atomic number, same
  protons, different neutrons, different mass
• Q Why do isotopes have different mass numbers
  for the same element?
• A Because they have different numbers of neutrons

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• The mass of an electron is insignificant
• Thomsons experiment with the gas tube detected
  electrons
• Rutherford discovered the positive central core
• The mass of each proton or neutron equals one
  atomic mass unit (amu)
• Q What is the charge of a proton? (neutron?
  electron?)
• Q Where is the proton located in an atom?
  (neutron? electron?)

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Atoms of Different Elements

• Atoms of any given element ALWAYS have the same
  number of PROTONS
• Definition atomic number of an element equals
  the number of protons
• Atoms of different elements ALWAYS have different
  numbers of PROTONS
• The number of PROTONS determines which element
  you have

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• Definition mass number of an atom is the sum of
  the protons plus the neutrons
• Neutrons Protons Mass
• Q Why arent electrons considered in the mass
  number of an atom?
• neutrons mass - atomic
• Q What subatomic particle could be substituted
  for atomic number?
• Q What 2 values represent mass?

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4.3 Modern Atomic Theory

• Objectives
• 1. Explain what can happen when atoms gain or
  lose energy
• 2. Explain the model that scientists use to
  describe how electrons behave in atoms
• 3. Describe the most stable configuration of
  electrons in an atom

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Losing and Gaining Energy

• Since the time of Democritus Dalton the model
  of the atom has been revised
• In 1913, Niels Bohr theorized that atoms move in
  set paths around the nucleus like planets
  orbiting the sun
• The electron path is an electrons energy level
• Energy level any of the possible energies an
  electron may have in an atom

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• Electrons in atoms must gain energy to move to a
  higher energy level or lose energy to move to a
  lower energy level
• The amount of energy gained (absorbed) to go
  higher or energy lost (released) can be measured
• Each element has its own unique set of energy
  levels
• The element itself determines the amount of
  energy gained or lost
• Bohrs model does not explain all chemical
  observations

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How Electrons Behave in Atoms

• Definition an electron cloud is a visual model
  of the most likely locations for electrons in an
  atom
• Scientists use the electron cloud model to
  describe the possible locations of electrons
  around the nucleus
• Instead of set paths like orbits, electrons are
  found in orbitals which have unique shapes

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• An electron cloud is a good approximation of how
  electrons behave in their orbitals
• Definition an orbital is a region of space
  around the nucleus of an atom where there is a
  high probability of finding an electron
• The level with the least energy has only one
  orbital
• Higher energy levels have more than one orbital

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Electron Configurations

• Definition an electron configuration is the
  arrangement of electrons in the orbitals of an
  atom
• The most stable configuration is the one in which
  the electrons are in orbitals with the lowest
  possible energies
• Definition the ground state is when all
  electrons in an atom have the lowest possible
  energies
• Definition excited state is when an electron is
  in an orbital with higher energy less stability