Unit II: Matter

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Unit II Matter

• Atomic Model Theory

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Key Concepts

• All matter is made up of atoms. Atoms are far
  too small to see with a light microscope.
• The atoms of any one element are different from
  the atoms of other elements.
• The periodic table is one useful model for
  classifying elements. The periodic table can be
  used to predict properties of elements. (metals,
  nonmetals, noble gases)

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The GREEK Model of Matter

• The Greek philosopher, Democritus, was given
  credit for naming the atom
• Four basic elements were earth, air, water and
  fire

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DALTONS Model
In 1803, an English chemist John Dalton proposed
an atomic theory of matter which included these
basic ideas

• All elements are composed of atoms. Atoms are
  indivisible and indestructible particles.
• Atoms of the same element are exactly alike.
• Atoms of different elements are different.
• Compounds are formed by the joining of atoms of
  two or more elements.

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THOMSONS Model

• In 1897,another English scientist J.J. Thomson
  revised the atomic theory. Through observation
  he reasoned that the atom was divisible (could be
  broken into smaller pieces).
• In his model, an atom was made of a puddinglike
  positively charged material throughout which
  negatively charged electrons were scattered,
  like plums in a pudding.

CATHODE RAY TUBE
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Plum Pudding Model of Atom
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RUTHERFORDS Model

• In 1908, another English scientist Ernest
  Rutherford developed an experiment in which
  further revision of the atomic theory would
  result.
• His experiment involved firing positively charged
  particles at a thin gold sheet (foil).

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Gold Foil Experiment

• In Rutherfords experiment, he thought that all
  of the positive particles would pass right
  through the gold sheet. He instead discovered
  that most did, pass through without a change in
  their path. But some were slightly deflected
  and even fewer bounced straight back.

Rutherford reasoned that most of the atom was
empty space and that it must have a very small,
dense, positively charged center. He called this
center the nucleus. He placed the electrons
around the atoms edge.
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Neils Bohr

• In 1913, a Danish scientist Neils Bohr improved
  the atomic model.
• Bohr placed the electrons in definite orbits
  around the nucleus.
• These orbits, he called energy levels and are
  located at certain distances from the nucleus.

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BOHRS Model

• The nucleus is in the center and electrons orbit
  it like planets around the Sun.
• When electrons go from one energy level to the
  next energy is either taken in or given off.

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WAVE Model

• The modern atomic model is based on the models of
  Rutherford and Bohr, and the principle of wave
  mechanics.
• The wave model states that the atom has a very
  small, dense and positive nucleus surrounded by
  negative electrons.
• The electrons are placed in clouds rather than
  orbits, and the cloud represents where the
  electron is most likely to be.

Electron clouds shown here represent where the
electrons may be. The more dense (dark) the cloud
the better chance that the electron is there.
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Structure of the Atom

• Particles smaller than an atom are called
  subatomic particles.
• The three main subatomic particles are proton,
  neutron and electron.