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Studying Atoms


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Title: Studying Atoms

Atomic Theory
  • Studying Atoms

Studying Atoms
  • How is it possible for us to study something we
    cant see?
  • Need indirect evidence
  • Atoms are so small that even with the most
    powerful microscopes we cant see them

Ancient Greek Models
  • Democritus believed that all matter consisted of
    small particles that couldnt be divided. He
    called these atoms from the Greek word atmos
    which means uncut or indivisible
  • Democritus thought there were different types of
    atoms with different types of properties atoms
    in liquids were round and smooth, atoms in solids
    were rough and prickly

Ancient Greek Models
  • Aristotle didnt think there was any limit to the
    number of times matter could be divided
  • Aristotle thought all matter was built up from
    four basic substances earth, air, fire, and
  • This idea was accepted up until the 1800s (over
    2000 years)
  • By the 1800 evidence to support an atomic model
    began to appear

Aristotles model of matter
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Daltons Atomic Theory
  • Dalton was interested in predicting the weather
    so he studied the behavior of gases in air
  • Predicted that a gas consisted of individual
    particles because of the pressure they exert

Daltons Evidence
  • When elements combine the ratio of the masses of
    the elements in the compounds is always the same
  • Compounds have a fixed composition

Daltons Theory
  • 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
  • Compounds contain atoms of more than one element
    (is this always true?)
  • In a particular compound, atoms of different
    elements always combine in the same way

Daltons Theory
  • His model of the atom pictured the atom as a
    solid sphere with different masses
  • Reminder A theory explains the results from many
  • Was widely accepted for a long time
  • Eventually scientists found that not all of
    Daltons ideas were completely correct
  • This lead to a revision of the theory

Daltons Model
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Thomsons Model of the Atom
  • Used an electric current to learn more about
  • Vacuum tube attached to an electric current a
    glowing beam appears
  • Beam contained charged particles
  • Used charged plates on either side of the beam
    beam attracted to positive plate and repelled
    from the negative plate

Evidence for subatomic particles
  • Particles in the beam had a negative charge
  • His experiments showed that atoms are made of
    smaller particles (Contrary to Daltons idea of a
    solid ball)

Thomson Model
  • Atoms are basically neutral no positive or
    negative charge
  • Negative charges were evenly scattered throughout
    an atom filled with a positively charged mass of
  • Plum pudding model (Chocolate Chip Ice Cream

Thomsons Model
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Rutherfords Atomic Theory
  • Discovered that uranium emitted fast-moving
    particles that had a positive charge (He called
    them alpha particles)
  • Gold Foil Experiment
  • Aimed the alpha (positive particles) at a thin
    gold foil
  • They did not behave as Rutherford expected

Rutherford/Marsden Experiment
Discovery of the nucleus
  • Rutherford proposed that the positive charge was
    concentrated in a small area which he called the
  • Nucleus a dense, positively charged mass located
    in the center of the atom

Rutherfords New Proposal
  • All of an atoms positive charge is concentrated
    in the nucleus
  • Through experimentation they discovered that the
    total volume of an atom is about a trillion
    (1,000,000,000,000) times the volume of the

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The Bohr Model
  • Model looks like planets orbiting around a sun in
    a solar system
  • Bohr agreed with Rutherford about the nucleus
  • Focused on the electrons
  • A description of the arrangement of the electrons
    is the main part of modern atomic theory

The Bohr Model
Energy Levels
  • Electrons move with constant speed in a fixed
    orbit around the nucleus
  • Each electron has a fixed amount of energy
  • Electrons can gain or lose energy
  • Energy levels are the possible energies that an
    electron can have
  • Like steps you cant be between you must be
    on one level or the other each step would
    represent an energy level
  • No two elements have the same set of energy levels

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Energy Levels
  • Electrons move from one energy level to another
    when they gain or lose energy
  • Can move more than one energy level
  • Evidence the light given off by fireworks
  • Raising energy levels and light given off when
    they return to their original energy level
  • Each element has a unique color spectrum they
    give off because each element has a different set
    of energy levels
  • Move from the ground state get excited return
    to the ground state and give off unique light

Structure of an Atom
Properties of Subatomic particles
  • Rutherford saw the existence of two subatomic
    particles and predicted a third.
  • Protons
  • Electrons
  • Neutrons

  • Rutherford concluded the the amount of positive
    charges varied from element to element
  • Each nucleus must contain at least one positive
    particle which he called a proton
  • Has a 1 charge
  • Some nuclei contain more than 100 protons

  • An electron is a negatively charged subatomic
    particle found in the space outside the nucleus
  • Electrons have a charge of -1

  • Neutrons were shown to exist by James Chadwick in
  • Charged objects did not deflect their paths
  • A neutron is a neutral subatomic particle that is
    found in the nucleus of an atom
  • It has a mass almost exactly equal to that of a
    proton (See page 109 in your textbook)

Comparing Subatomic Particles
  • All three subatomic particles can be
    distinguished by mass, charge, and their location
    in an atom
  • All of this information is based on how particles
    behave. We still dont have any instrument that
    can see inside an atom.

Atomic Number
  • Atomic Number is the number of protons in the
    nucleus of an atom
  • Atoms of different elements have different atomic
  • The atomic number also equals the number of
    electrons. Why?

Mass Number
  • The mass number is the sum of the number of
    protons plus the number of neutrons
  • You can figure out the number of neutrons by
    subtracting the atomic number from the mass
  • Al 27 How many neutrons?
  • Carbon 12 How many neutrons?
  • Argon 40 How many neutrons?

  • Every atom of the same element does not have the
    same number of neutrons
  • Isotopes are atoms of the same element that have
    different numbers of neutrons and therefore
    different mass numbers
  • Radioactive elements typically are isotopes
  • Some others are oxygen and carbon
  • All of the elements have isotopes

Modern Atomic Theory
Electron Cloud Model
  • An improvement on Bohrs model
  • Electrons move in a less predictable way
  • Electron Cloud a visual model of the most likely
    locations for the electrons of an atom to be
    around the nucleus
  • Cloud is denser where the electrons are most
    likely to be based on probability

Electron Cloud Model
Atomic Orbitals
  • An orbital is a region of space around the
    nucleus where an electron is likely to be found
  • These orbitals have unique shapes and can hold 2
    electrons each
  • s, p, d, and f
  • s orbitals (there are 1) can hold 2 electrons
  • p orbitals (there are 3) can hold 6 electrons
  • d orbitals (there are 5) can hold 10 electrons
  • f orbitals (there are 7) - can hold 14 electrons

Energy Level Number of Orbitals Possible Maximum of Electrons
1 1 2
2 4 8
3 9 18
4 16 32
Electron Configuration
  • 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 orbitals with the
    lowest possible energies (we call this the ground
  • If an electron absorbs energy it is said to be in
    an excited state which is less stable (light