Chapter 3 Section 1 Notes

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Chapter 3 Section 1 Notes

• Atomic Structure and History of the Atomic Theory

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Do theories in science always stay the same?

• No, theories are changed or replaced as new
  discoveries are made.
• Theories of the atom (Atomic Theory) have changed
  many times over the past couple thousand years
  based on new knowledge that has been gained
  through experimenting.
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What are Atoms?

• Atoms are tiny units that determine the
  properties of matter
• Atoms are EVERYWHERE and make up EVERYTHING
  around you!

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Democritus (460-370 BC)

• In the 4th Century B.C., the Greek philosopher
  Democritus suggested that the universe was made
  of indivisible units that he called atoms.
• Atom comes from the Greek word atomos, which
  means unable to be cut or divided

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Democritus (460-370 BC)

• Democritus concluded that there was a limit to
  how far matter could be divided eventually, you
  would end up with a piece of matter that could
  not be cut.
• What did people think of Democritus? People did
  not believe Democritus because he had no evidence
  to support his theory.

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John Dalton (1766-1844)

• In 1808, two thousand years after Democritus, an
  English school teacher named John Dalton proposed
  a revised atomic theory.
• His theory was developed based on scientific
  evidence and some parts of his theory still hold
  true today!

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John Dalton (1766-1844)

• John Dalton proposed 3 new ideas about the atom
• Every element is made of tiny, unique particles
  called atoms that cannot be subdivided.
• Atoms of the same element are exactly alike.
• Atoms of different elements can join to form
  molecules.
  
  
  

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Evidence for Dalton

• Unlike Democritus, Dalton based his theory on
  experimental evidence.
• Today, Daltons theory is considered the
  foundation for the modern atomic theory.
• Some parts of this theory turned out to be
  correct, but his theory could not explain all of
  the experimental evidence.

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J.J. Thomson (1856-1940)

• In 1897, J.J. Thomson, a British scientist,
  performed an experiment that suggested that atoms
  were not indivisible.

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J.J. Thomson (1856-1940)

• What did Thomson discover?
• While investigating with electricity and cathode
  ray tubes, Thomson discovered that atoms are made
  of smaller, negatively charged particles called
  electrons.

The stream of electrons is attracted to
positively charged plate.
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Thomsons Atomic Theory

• In his model, electrons are spread throughout an
  atom, just like blueberries are spread throughout
  a muffin.
• His model is called the plum-pudding model, which
  was a dessert popular in his time.

In this model, electrons are scattered throughout
the atom.
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Ernest Rutherford (1871-1937)

• Was a student of J.J. Thomson but disagreed with
  the Plum Pudding Model.
• Devised an experiment to investigate the
  structure of positive and negative charges in the
  atom.

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What did Rutherford Propose?

• Rutherford proposed that most of the mass of the
  atom was in the atoms center.
• An interactive model of Rutherfords Gold-foil
  Experiment
• http//micro.magnet.fsu.edu/electromag/java/ruther
  ford

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Rutherfords Gold-Foil Experiment

• What did most of the alpha particles shot at the
  gold foil do?
• Most of the particles traveled straight through
  the gold foil.
• What was the surprising behavior of a few of the
  particles?
• A few of the particles were deflected and some
  even bounced back.
• http//cwx.prenhall.com/petrucci/medialib/media_po
  rtfolio/text_images/006_RUTHERFORD.MOV

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Rutherfords Atomic Model

• What did Rutherford discover?
• Rutherford discovered the nucleus, which is the
  dense core of an atom.
• What is Rutherfords Atomic Model?
• Negatively charged electrons orbit the nucleus
  like planets orbit the sun.

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Niels Bohr (1885-1962)

• In 1913, the Danish scientist Bohr suggested that
  electrons in an atom move in set paths around the
  nucleus like planets orbiting the sun.
• In his model, every electron has a certain energy
  level that is determined by its path around the
  nucleus.

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Modern Atomic Theory (1925)

• This is the theory that we currently believe.
• The modern atomic theory believes that electrons
  behave more like waves on a vibrating string.

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Do atoms really look like this?

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Well..

• It is a much easier way to fit it on paper!
• If a nucleus were as big as a marble, the whole
  atom would be the size of a football stadium!
  The nucleus is very, very small in comparison to
  the atom.

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So, can atoms be subdivided?

• Both Democritus in the 4th century and later
  Dalton in the 19th century believed that the atom
  was the smallest particle and could not be
  subdivided.
• We now know that this is NOT TRUE!!!!- the atom
  can be divided into subatomic particles

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Whats in an atom?

• An atom has 3 subatomic particles in it
• Protons have a positive charge and are located
  in the nucleus
• Neutrons have no charge and are located in the
  nucleus

• Electrons have a negative charge and are located
  orbiting around outside the nucleus

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Parts of an Atom
Particle Location Mass Charge
Proton Nucleus 1.67 x 10-27 kg 1
Neutron Nucleus 1.67 x 10-27 kg 0
Electron Outside the nucleus 9.11 x 10-31 kg -1
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Overall Charge of an Atom

• The overall charge of an atom is ZERO. Atoms
  have equal numbers of protons and electrons, and
  their charges exactly cancel.

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The nucleus

• What two particles are located in the nucleus?
• Protons and Neutrons
• What is the overall charge of the nucleus ONLY?
• The nucleus has a positive charge because it is
  made of protons, which are positive, and
  neutrons, which have no charge.

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Energy Levels

• According to Modern Atomic Theory, it is nearly
  impossible to determine the exact location of an
  electron.
• 1st Energy Level can hold up to 2 e-
• 2nd Energy Level can hold up to 8 e-
• 3rd Energy Level can hold up to 18 e-
• Note Each lower level needs to be filled before
  going to the next energy level.

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Orbitals

• An orbital is a region in an atom where there is
  a high probability of finding an electron.
• There are 4 orbitals
• s orbital
• p orbital
• d orbital
• f orbital
• These orbitals are in order from lowest to
  highest energy.

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Valence Electrons

• A Valence Electron is an electron that is located
  in the outer most energy level.