THE ATOM

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THE ATOM

• Part 1

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The Atom
From Philosophy to Science

• Think with meabout sugar crystals, you can see
  that they are small crystals and every crystal is
  identical.
• You may grind these particles into a very fine
  powder, but each tiny piece is still sugar.
• If you dissolve the sugar in water, the sugar
  particles become virtually invisible.

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The Atom
From Philosophy to Science

• You could look at the dissolved sugar water under
  a microscope and youd still not be able to see
  the sugar.
• However, you know it is still there because you
  can taste it.
• These kind of observations and logic patterns
  led ancient philoso- phers to ponder the design
  of the universe

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• The two schools of thought philoso-phers were
  divided into wereis everything in the universe
  continuous and infinitely divisible, or is there
  a limit to how small you can get?
• Particle theory was not a popular early
  opinion, but was supported as early as
  Democritus in ancient Greece.

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• Democritus proposed that all the matter is
  composed of tiny particles called Atomos
• These particles are indivisible
• Aristotle did not believe in Democ-ritus atom,
  he was of the matter is continuous philosophy
• Because of Aristotles popularity his idea was
  accepted as the better philosophy for 200 years

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• In the 1700s nearly all chemists accepted the
  modern definition of an element as a substance
  that is indivisible
• It was also thought that elements combine to form
  compounds that are different in their
  properties than the elements that composed
  them

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• There was controversy as to whether elements
  always combine in the same ratio when forming a
  particular compound.
• In the 1790s, chemistry was revolut-ionized by a
  new emphasis on quant- itative analysis because
  of new and improved balances

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• This new technology led to the discovery of some
  new scientific laws
• The Law of Conservation of Mass
• States that mass is neither created nor destroyed
  during ordinary chemical rxns or physical
  changes.
• Which means the total mass of the reactants must
  equal the total mass of the products.

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Law of Conservation of Mass

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• The Law of Definite Proportions
• The fact that a chemical compound contains the
  same elements in exactly the same proportions by
  mass regardless of the size of the sample or the
  source of the compound
• NaCl is NaCl no matter if it is table salt (small
  crystals) or rock salt (large crystals)

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Law of Definite Proportions
ALWAYS


1 Carbon
1 Oxygen
Carbon Monoxide
ALWAYS

Carbon Dioxide, CO2
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• The Law of Multiple Proportions
• If 2 or more different compounds are composed of
  the same 2 elements, then the ratio of the masses
  of the 2nd element combined with a certain mass
  of the 1st element is always a ratio of small
  whole numbers

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Law of Multiple Proportions


Carbon
Oxygen
Carbon Monoxide, 11
1
1

Carbon Dioxide, 12
1
2
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• In 1808, John Dalton proposed an explanation for
  each of the proposed laws
• He reasoned that elements were composed of atoms
  that only whole s of atoms can combine to
  form compnds
• His ideas are now called the Atomic Theory of
  Matter

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ELEMENT 2
ELEMENT 3
ELEMENT 4
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• Through these statements, evidence could be
  gathered to confirm or discount its claims
• Not all of Daltons claims held up to the
  scrutiny of experimentation
• Atoms CAN be divided into even smaller
  particles
• Not every atom of an element has an identical mass

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• Daltons Atomic Theory of Matter has been
  modified.
• What remains is
• All matter is composed of atoms
• Atoms of any one element differ in properties
  from atoms of another element

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The Structure of the Atom

• One of the disputed statements of Dalton was that
  atoms are indivisible
• In the 1800s it was determined that atoms are
  actually composed of several basic types of
  smaller particles
• its the number and arrangement of these
  particles that determine the atoms chemical
  properties.

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The Structure of the Atom

• The def. of an atom that emerged was, the
  smallest particle of an element that retains the
  chemical properties of that original element.
• All atoms consist of 2 regions that contain the
  subatomic particles
• The nucleus
• The electron cloud around the nucleus

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The Structure of the Atom

• The nucleus is a very small region located near
  the center of the atom
• In every atom the nucleus contains at least 1
  proton, which is positively charged particle
• Usually contains 1 or more neutral
  particles called neutrons

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The Structure of the Atom

• The electron cloud is the region that surrounds
  the nucleus
• This region contains 1 or more electrons, which
  are negatively charged subatomic particles
• The volume of the electron cloud
  is much larger than the
  nucleus

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The Structure of the Atom

• The 1st discovery of a subatomic particle took
  place in the late 1800s.
• electric current was passed through various gases
  at low pressures
• The glass tubes the experiments were
  carried out in are called cathode
  ray tubes.

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The Structure of the Atom

• Investigators noticed that when electric current
  was passed through a cathode ray tube, the
  surface of the tube directly opposite the cathode
  glowed.
• they decided that the glow was caused by a
  stream of particles
• they called the stream a cathode ray

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cathode ray
cathode
Cathode Ray
anode
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The Structure of the Atom

• The ray traveled from the cathode to the anode
  when current was passed through the tube
• Electric current instead flows from the anode to
  the cathode
• Scientists began to cleverly study cathode
  rays and observed several remarkable
  characteristics.

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The Structure of the Atom

• An object placed in the path of the ray cast a
  shadow on the glass
• A paddle wheel placed the path of the cathode ray
  began to spin
• Cathode rays were deflected by a magnetic field
• The rays were deflected away from a negatively
  charged object

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The Structure of the Atom

• The first 2 observations support the evidence
  that the cathode ray is composed of particles
  traveling through the depressurized gases
• The second 2 observations support the evidence
  that the ray is composed of a substance that
  has a negative charge to it.

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magnet
cathode ray
deflection due to magnetic field
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The Structure of the Atom

• An English Physicist J.J. Thomson became the
  authority on cathode-rays
• He measured the ratio of the charge of
  cathode-ray particles to their mass
• He discovered the ratio was always the same
  regardless of the conditions

His conclusion all cathode-rays are composed of
identical negatively charged particles a.k.a.
electrons
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The Structure of the Atom

• Thomsons experiments revealed that the electron
  has a very large charge in relation to its mass
• In 1909, Robert Millikan, performed an ingenious
  experiment to calculate the mass of an electron
• he calculated that the electrons mass is
  9.109x10-31kg

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The Structure of the Atom

• Thomsons Millikans Ideas
• atoms are composed of smaller particles, and one
  of these compo-nents is negatively charged
• atoms are neutral, so there must be an opposing
  () charge
• because Es are essentially mass-less there must
  be something else that accounts for the atoms
  mass.

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The Structure of the Atom

• These proposals led to an elemen-tary
  understanding of the compo-sition of the atom.
• The atom might look something like a tiny
  chocolate chip cookie

The chips would be the Es
The cookie part is () matter to cancel out the
(-) chips
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The Structure of the Atom

• In 1886, E. Goldstein observed a cathode-ray tube
  and found rays traveling in the opposite
  direction of that of the cathode rays
• The new rays are called canal rays and they are
  () charged
• And the particles mass is about 2000 Xs that of
  the electron

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The Structure of the Atom

• In 1932, the English physicist James Chadwick
  discovered yet another subatomic particle.
• the neutron has no electrical charge
• Its mass is nearly equal to the proton
• Therefore the subatomic particles are the
  electron, proton, and neutron.

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The Structure of the Atom

• When subatomic particles were discovered,
  scientists wondered how they were put together in
  an atom.
• This was a difficult question to resolve, given
  how tiny atoms are.
• Most thought it likely that the atom resembled
  the chocolate chip cookie we discussed earlier

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The Structure of the Atom

• In 1911, Ernest Rutherford et al. provided a more
  detailed picture of the internal structure of the
  atom
• In his experiment, Rutherford direct-ed a narrow
  beam of alpha particles at a very thin sheet of
  gold foil.
• Alpha particles (?) are He atoms that have been
  stripped of its electrons

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The Structure of the Atom

• According to the prevailing theory, the alpha
  particles should have passed easily through the
  gold, with only a slight deflection
• And mostly thats how it happened.
• However, they found 1 every 8000 particles had
  actually been deflected back toward the source.

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The Structure of the Atom

• Based on the results, Rutherford suggested a new
  theory of the atom.
• The atom is mostly empty space
• He concluded that all the () charge and almost
  all the mass is conc. in a small core in the
  center of the atom
• The Es surround the positively charged nucleus
  like planets surround the sun.

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The Structure of the Atom

• With the exception of Hydrogen, all nuclei
  contain 2 kinds of particles protons and neutrons
• Proton has a () charge equal in mag-nitude to
  the (-) charge of an electron
• Atoms are neutral because they con- tain equal
  s of protons electrons
• A neutron is electrically neutral

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The Structure of the Atom

• The nuclei of atoms of different elements differ
  in the of protons they contain and therefore in
  the amount of () charge they possess.
• The of protons the atom contains determines the
  atoms identity
• Oxygen contains 8 protons
• Fluorine contains 9 protons
• Neon contains 10 protons

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Properties of Subatomic Particles
RELATIVE
RELATIVE
ACTUAL
MASS (g)
SYMBOL
PARTICLE
MASS
CHARGE
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The Structure of the Atom

• The nucleus is composed of a coll-ection of
  protons, which are all ()
• Shouldnt they repel each other like charges and
  all?
• When 2 protons are very close to each other,
  there is a strong force of attraction between
  them.
• A similar attraction exists when neutrons are
  close too.

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The Structure of the Atom

• These short-range p-n0, p-p, and n0-n0 forces
  hold the nuclear particles together, A.K.A
  nuclear forces.
• When these nuclear forces are strong enough the
  atom is stable
• If the forces are not strong enough the atom
  (heavier atoms) the atom is unstable and
  becomes radioactive.

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The Structure of the Atom

• The radius or size of the atom can be determined
  through the volume of space the electron(s)
  occupy, A.K.A. the electron cloud.
• The radius of the atom is the distance from the
  center of the nucleus to the outer portion of
  the electron cloud.
• Atomic radii are expressed in picometers (pm)

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The Structure of the Atom

• Atomic radii range from about 40 to 270 pm.
• The nuclei of atoms have much smaller radii,
  about 0.001 picometers

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