Atomic Theories (Sections 4.1

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Atomic Theories(Sections 4.1 4.3)
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Ancient Greek Models

• Democritus believed that all matter consisted of
  extremely small particles that could not be
  divided
• Atoms
• Means uncut or indivisible
• Thought there were different types of atoms with
  specific properties
• Smooth and round for liquids, rough and prickly
  for solids

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Ancient Greek Models

• Aristotle didnt think there was a limit to the
  number of times matter could be divided
• Thought that all substances were built up from
  only four elements
• Earth
• Air
• Fire
• Water
• His theory was accepted for many centuries

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Daltons Atomic Theory

• Gathered evidence by measuring masses when
  elements combine and form compounds
• Developed a theory to explain why the elements in
  a compound always join in the same way
• Elements are like solid spheres
• Made wooden spheres to represent the atoms of
  different elements

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Daltons Theory

• Dalton proposed the theory that all matter is
  made up of individual particles called atoms,
  which cannot be divided
• All elements are composed of atoms
• All atoms of the same element have the same mass,
  and atoms of different elements have different
  masses
• Compounds contain atoms of more than one element
• In a particular compound, atoms of different
  elements always combine in the same way

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Thomsons Model of the Atom

• Used an electric current to learn more about
  atoms
• Electric current is a flow of charged particles
• Objects with like charges repel and objects with
  opposite charges attract
• Used a sealed tube of gas with two oppositely
  charged disks on either side
• Cathode Ray Tube
• A glowing beam appeared between the two disks
• The beam bent toward a positively charged plate
  outside the tube

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Thomsons Model

• The particles inside the beam were negatively
  charged and came from inside atoms
• Thomsons experiments provided the first evidence
  that atoms are made of even smaller particles
• Revised Daltons theory to account for these
  subatomic particles

• Thomsons Model
• Plum Pudding model
• Negative charges are evenly scattered throughout
  an atom filled with a positively charged mass of
  matter
• Like chocolate chip ice cookie

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Rutherfords Atomic Theory

• Gold Foil Experiment
• Used alpha particles and a screen of gold
• Found that many particles were deflected more
  than 90 degrees, some straight back

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Rutherfords Model

• Found that the positively charge of an atom is
  not evenly spread throughout the atom
• It is concentrated in a very small central area
  called the nucleus
• 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

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Bohrs Model of the Atom

• Focused on the electrons
• In Bohrs model, the electrons move with constant
  speed in fixed orbits around the nucleus
• Like planets around the sun
• Each electron has a specific amount of energy and
  if it gains or loses energy the energy of the
  electron can change
• Energy levels - the possible energies that
  electrons in a atom can have

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Energy Levels

• Like steps in a staircase
• As you move up or down you can measure how many
  steps you took
• You can take one step up or jump up two steps
  down
• Up or down you can only move in whole-step
  increments, never between steps
• An electron in an atom can move from one energy
  level to another when the atom gains or loses
  energy
• Can move up two energy levels if it gains enough
  energy
• Can move down two energy levels if it loses
  enough energy
• The size of the jump determines the amount of
  energy gained or lost

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Evidence for Energy Levels

• Scientists can measure the energy gained when
  electrons absorb energy and move to a higher
  energy level
• They can measure the energy released when the
  electron returns to a lower energy level
• The movement of electrons between energy levels
  explains the light you see when fireworks explode
• Heat from the explosion causes some electrons to
  move to higher energy levels, when they move back
  they emit some energy as visible light
• Different elements emit different colors of light

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Electron Cloud Model

• Electrons move in a less predictable way than
  Bohr thought
• Electron cloud - a visual model of the most
  likely locations for electrons in an atom
• Denser at locations where the probability of
  finding an electron is high
• Scientists use the electron cloud model to
  describe the possible locations of electrons
  around the nucleus

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Atomic Orbitals

• The electron cloud represents all the orbitals in
  an atom
• Orbital - a region of space around the nucleus
  where an electron is likely to be found
• Electron cloud is a good approximation of how
  electrons behave in their orbitals

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Energy Levels, Orbitals, Electrons

• The level in which an electron has the least
  energy (lowest energy level) has only one orbital
• Higher energy levels have more than one orbital
• The first four energy levels of an atom
• Max of electrons in an energy level is twice
  the number of orbitals
• Each orbital can contain two electrons at most

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Electron Configurations

• Electron configuration - the arrangement of
  electrons in the orbitals of an atom
• The most stable electron configuration is the one
  in which the electrons are in the orbitals with
  the lowest possible energies
• Ground state - when all the electrons in an atom
  have the lowest possible energies
• Excited state - when an electron is in an orbital
  with a higher energy level, less stable than the
  ground state