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The atoms and the atomic theory


The atoms and the atomic theory The Beginning of the Atomic Theory What Is an Element? Around 440 BCE, a Greek philosopher named Democritus thought that you would ... – PowerPoint PPT presentation

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Title: The atoms and the atomic theory

The atoms and the atomic theory
The Beginning of the Atomic Theory
  • What Is an Element? Around 440 BCE, a Greek
    philosopher named Democritus thought that you
    would eventually end up with a particle that
    could not be cut. He called this particle an
  • From Aristotle to Modern Science Aristotle,
    another Greek philosopher, disagreed with
    Democrituss ideas. He believed that you would
    never end up with a particle that could not be

The Beginning of the Atomic Theory
  • From Aristotle to Modern Science Democritus was
    right, though Matter is made of particles, which
    we call atoms. An atom is the smallest particle
    into which an element can be divided and still be
    the same substance.

Daltons Atomic Theory Based on Experiments
  • Daltons Theory John Dalton published his atomic
    theory in 1803. His theory stated that all
    substances are made of atoms. Atoms are small
    particles that cannot be created, divided, or
    destroyed. Atoms of the same element are exactly
    alike, and atoms of different elements are
    different. Atoms join with other atoms to make
    new substances.
  • Not Quite Correct The atomic theory was then
    changed to describe the atom more correctly.

Thomsons Discovery of Electrons
  • Negatively Charged Particles Thomson
    experimented with a cathode-ray tube like the one
    shown on the next slide. He discovered negatively
    charged particles that are now known as
  • Like Plums in Pudding After learning that atoms
    contain electrons, Thomson proposed a new model
    of the atom. Thomson thought that electrons were
    mixed throughout an atom, like plums in a

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Rutherfords Atomic Shooting Gallery-
discovered the nucleus
  • Negatively Charged Particles In 1909, Ernest
    Rutherford aimed a beam of small, positively
    charged particles at a thin sheet of gold foil.
    The next slide shows his experiment.
  • Surprising Results Rutherford expected the
    particles to pass right through the gold in a
    straight line. To Rutherfords great surprise,
    some of the particles were deflected.

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Where Are the Electrons?
  • Far from the Nucleus Rutherford proposed that in
    the center of the atom is a tiny, positively
    charged part called the nucleus.
  • Bohrs Electron Levels In 1913, Niels Bohr
    proposed that electrons move around the nucleus
    in certain paths, or energy levels.

Where Are the Electrons?
  • The Modern Atomic Theory According to the
    current theory, there are regions inside the atom
    where electrons are likely to found. These
    regions are called electron clouds (Proposed by
    Schrodinger 1926)

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How Small Is an Atom?
  • Three One-Hundred-Millionths of a Centimeter!
    Scientists know that aluminum is made of
    average-sized atoms. An aluminum atom has a
    diameter of about 0.00000003 cm.
  • Even smaller than the wavelengths of visible

What Is an Atom Made Of?
  • The Nucleus Protons are positively charged
    particles in the nucleus. Neutrons are the
    particles of the nucleus that have no electrical
  • Outside the Nucleus Electrons are the
    negatively charged particles in atoms. Electrons
    are found around the nucleus within electron
    clouds. Define the volume of an atom. All the
    structures of the atom can be seen on the next

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How Do Atoms of Different Elements Differ?
  • Starting Simply The hydrogen atom has one proton
    and one electron.
  • Now for Some Neutrons The helium atom has two
    protons, two neutrons, and two electrons.

How Do Atoms of Different Elements Differ?
  • Building Bigger Atoms For bigger atoms, simply
    add protons, neutrons, and electrons.
  • Protons and Atomic Number The number of protons
    in the nucleus of an atom is the atomic number of
    that atom. All atoms of an element have the same
    atomic number (Z)

  • Isotopes are atoms that have the same number of
    protons but have different numbers of neutrons.

  • Properties of Isotopes An unstable atom is an
    atom with a nucleus that will change over time.
    This type of isotope is radioactive.
  • Telling Isotopes Apart You can identify each
    isotope of an element by its mass number. The
    mass number (A) is the sum of the protons and
    neutrons in an atom.

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  • Naming Isotopes To identify a specific isotope
    of an element, write the name of the element
    followed by a hyphen and the mass number of the
  • Calculating the Mass of an Element The atomic
    mass of an element is the weighted average of the
    masses of all the naturally occurring isotopes of
    that element.

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Forces in Atoms
  • Four Basic Forces Four basic forces are at work
    everywhere, even within the atom. These forces
    are gravitational force, electromagnetic force,
    strong force, and weak force.
  • These forces work together to give an atom its
    structure and properties.

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