The Atom

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The Atom

• Mr. Sackman South Dade Senior High
• 2010

2
History

• The Greeks
• Dalton
• J.J. Thompson
• Millikan
• Rutherford
• Chadwick
• Bohr

3
History

• The Greeks were the first to attempt to describe
  matter and atoms
• Philosophers were intellectual thinkers of the
  time and anything that they said many believed
  without argument
• The Greeks first classified matter as Earth,
  Wind, Water, and Fire
• Their ideas were creative, however, there was no
  way to test their theories at the time another
  reason many just accepted what philosophers said
• They believed that matter could endlessly,
  meaning infinitely, be divided into smaller and
  smaller pieces with no end

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History

• Democritus
• First to propose that matter isnt infinitely
  divisible
• Believed matter was made of tiny particles called
  atomos (atoms)
• Believed atoms could not be created, destroyed,
  or further divided
• Matter is composed of empty space through which
  atoms move

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History

• Atoms are solid, homogenous, and indivisible
• Different kinds of atoms have different shapes
  and sizes
• The differing properties of matter are due to the
  size, shape, and movement of atoms
• Changes in matter can only be caused by changes
  in grouping of atoms and not from changes in the
  atoms themselves
• His thinking was way ahead of his time and some
  of his ideas still hold

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History

• Aristotle (384 B.C.-322B.C.)
• One of the most influential minds of his time
• Gained wide acceptance for his view on nature
• What ever he stated most accepted, or believed,
  to be fact or true
• He rejected atomic theory all together simply
  because of his own ideas didnt agree
• His major argument was that matter isn't empty
  space through which atoms move, he didnt believe
  nothingness could exist

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History

• Democritus was unable to answers challenges to
  his ideas paving the way for Aristotle's beliefs
• Democritus' ideas were then eventually thrown out
• Aristotles theory was accepted and he threw out
  the existence of atoms altogether
• Because of Aristotles influence the answer to
  the question of the acceptance, or denial, of
  atoms went unchallenged for 2000 years

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History

• John Dalton (1766-1844)
• Finally thousands of years later someone
  attempted to describe the atom
• He marked the beginning of modern atomic theory
• Science now allowed for the study of matter and
  attempted to prove the existence of atoms
• Proposed new atomic theory in 1803
• Some of his theories were the same as Democritus

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History

• -Daltons Atomic Theory states the following
• -All matter is composed of extremely small
• particles called atoms
• -All atoms of the same element are identical and
  atoms of different elements differ completely
  from others
• -Atoms cannot be created, destroyed, or divided
• -Different numbers of atoms combine in simple
  whole number ratios to form compounds and in
  chemical reactions atoms are separated, combined,
  or rearranged

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History

• Recall the law of conservation of mass? Dalton's
  theory easily explains this law by stating that
  atoms are only separated or rearranged in
  reactions which would neither create or destroy
  atoms
• The law of definite proportions states that no
  matter how large the sample is a compound is
  always composed of the same elements in the same
  proportion by mass. For example water is always
  11 H and 89 O no matter how large or small the
  sample

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• Dalton had used some form of technology and the
  new aged science to refine Democritus' theory
• Dalton observed and recorded numerous reactions
  making careful observations, and measurements, as
  he performed his experiments, does this process
  sound familiar?

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History

• Using Lavoisier's, and Prousts, ideas he had
  come up with his own
• Is his theory completely correct?
• How did his theory differ from that of the
  Greeks?
• What do you think gave him the advantage of
  acceptance of his theory at that time?

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Defining the Atom

• What is the actual definition of an atom?
• What does an atom look like?
• Can you picture something that is so small you
  cant see?

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Defining the Atom

• Suppose you decide one day you want to grind your
  pure silver necklace down, how far could you
  grind?
• Could you eventually grind down far enough that
  you reach something that is not divisible or
  visible?
• Does every smaller and smaller piece you grind
  retain the properties of what you are grinding
  down?

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Defining the Atom

• Exactly how small is an atom?
• The book gives a very good example and it is
• Consider the size of the population in the world
  which in 2000 was about 6 billion or
  6,000,000,000 now compare that to how many Cu
  atoms are in a penny which is 29,000,000,000,000,0
  00,000,000
• This is almost 5 billion times more copper atoms
  than people

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Defining the Atom

• Now take the diameter of that same penny,
  1.20X10-10m
• If one were to place six billion copper atoms
  side by side, this is the same as the world
  population, the line of copper atoms would be
  less than the length of a meter stick

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Defining the Atom

• Can one actually see an atom with technology
  these days?
• Now can you begin to see, or picture, the size
  and existence of atoms?

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The electron

• Everyone here knows what an electron is, or do
  you?
• How do we prove the existence of an electron
• Did we set out to prove there was such a particle
  called the electron or was it accident?
• Curiosity sparked the investigation between
  electrical charge and matter
• By accident, one day, Henry Crookes noticed a
  flash of light from one of his tubes he created
  while working in a dark laboratory

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The electron

• These flashes were the result of something
  striking a light producing coating applied at one
  end of a cathode tube
• Further investigation showed that that a stream
  appeared to flow from the cathode to the anode
• This device led to the one of the most important
  social developments of all time, T.V. (old school
  ones) and computer monitors. (also old school
  ones) Pictures on these screens are just formed
  when radiation from the cathode strikes light
  producing chemicals that coat the backside of a
  screen producing an image

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The electron

• Research showed this stream of light was actually
  a ray of particles not just some invisible rays
• The particles were shown to carry a negative
  charge when a magnetic either deflected or
  attracted the stream. How could you prove this
  with only a magnet?

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The electron

• J.J. Thompson (1856-1940)
• Began a series of cathode ray tube experiments
  when using Crookes technology
• He calculated both the magnetic and electrical
  fields and found the mass to charge ratio
• When comparing this ratio to other known ratios
  he concluded that this charged particle was
  actually lighter than a hydrogen atom
• What did the mass of this particle being less
  than that of the smallest atom prove?

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The electron

• When changing the matter that filled the tube the
  results were the same, what does this mean?
• His theory went unaccepted for some time as many
  still believed the atom is indivisible, Daltons
  theory
• What did Thompson just prove and demonstrate, how
  important was this at this time?

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The Electron

• Millikan (1856-1940)
• Determined, and proved, the charge of an electron
  to be negative in 1909
• His technique, and set up, was so accurate that
  his value found in 1909 still only has an error
  of 1.0
• Charge was determined to be that of a single
  charge, meaning ,-1
• Now knowing the charge, and the mass to charge
  ratio discovered my Thompson, he calculated the
  mass to be 9.1 X10-28g which is 1/1840 the mass
  of a hydrogen atom what does this mean?

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The Proton

• Matter is in a electrically neutral state most of
  the time so how are atoms neutral if they carry a
  particle with a negative charge?
• There must be a positive charge as well
• J.J. Thompson proposes plum pudding model
  describes the atom to be a spherical shape of
  uniformly distributed positive charge spherically
  around the atom with the electrons packed inside.
  

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The Proton

• The plum pudding model doesnt hold for long when
  Rutherford comes around
• In 1911, Ernest Rutherford, simply interested on
  how alpha particles interacted with matter, began
  a series of experiments
• He conducted experiments to see if alpha
  particles were deflected if passed through a
  thing sheet of gold foil, this experiment was a
  breakthrough in atomic theory

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The Proton

• Knowing Thompsons model Rutherford expected only
  minor deflections of alpha particles
• Believed that if there was any deflection it
  would be due to the collision, or near collision,
  of a negatively charged electron
• He also believed that the positive charge was so
  uniform throughout it wouldn't deflect the
  massive alpha particles
• His results were stunning and opposite of what he
  expected

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• As you saw in the pictures of this gold foil
  experiment some of the rays went directly through
  the foil, some were deflected a little, and some
  deflected straight back to the source, what did
  this indicate?
• The ones that passed right trough were not
  interacting with the atom as they passed through
  empty space
• The rays deflected were due to the interaction of
  the positively charge nucleus of atoms, the
  closer the ray to the nucleus the great the
  deflection

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• He proposed that the atom was mostly empty space
  with a central, and extremely, densely packed
  nucleus which contained all the positive charge

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The Nucleus

• An atom is mostly empty space so how big, or
  small is an atom?
• We already studied the size of an atom, but these
  particles are smaller than the atom itself,
  aren't they?
• If the nucleus were the size of a dot on an
  exclamation point than the mass of that nucleus
  would be that of 70 automobiles
• If the atom had the diameter of a football field
  the nucleus would be the size of only a nickel
• What does this mean?

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The Neutron
Particle Symbol Location Relative electrical charge Relative Mass (amu) Actual Mass
Electron e- In the space around some place around the nucleus 1- 1/1840 9.11X10-28
Proton P Nucleus 1 1 1.673X10-24
Neutron N0 Nucleus 0 1 1.675X10-24

• Now putting all the results together we discover
  the mass of a electron, and that of a proton, did
  not account for the entire mass of the lightest
  known atom, what does this mean?
• The answer came with the discovery of the neutron
  
• James Chadwick, a student of Rutherford
  discovered another particle in the nucleus with
  no charge that had a mass equal to that of a
  proton both 1.675X10-24g, or 1 amu, much larger
  than the mass of an electron which is nearly
  equal mass to the mass of a proton, both
  subatomic particles are given a mass of 1amu

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How Atoms Differ

• Not long after Rutherford a man named, Henry
  Moseley, demonstrated that atoms of each
  different element had a unique positive charge in
  the nucleus, what does this mean?
• The number of protons defines, or identifies, the
  element and is called the atomic number(Z), each
  proton has a charge of 1, the opposite of the
  electron, and has a mass of 1amu
• Since the mass of a neutron, and proton, are
  equal and more massive than an electron both
  particles added together for the mass number(A)
  and the electron isn't

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How Atoms Differ

• How could on calculate the number of neutrons
  from these two numbers?
• If the mass number is equal to the number of
  protons plus neutrons, and the atomic number is
  the number of protons, all we have to do is
  subtract the atomic number from the mass number,
  A-Z
• What are the letters for each element on the
  periodic table, why is the first always
  capitalized and the second always lower case?

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Isotopes

• If the mass of a neutron, and a proton, both
  equal 1 amu why are the masses on the periodic
  table not whole numbers?
• The answer is because of isotopes
• When you take a sample of an element the sample
  contains the element plus any of its isotopes
• Isotopes are the same element but have different
  number of neutrons causing the masses to be
  different
• Why couldnt there be isotopes of the same
  element with different numbers of protons?

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Isotopes

• JJ Thompson discovered one day that he two
  separate samples of neon gas, the same element,
  but they had different masses, how could this be?
• The answer again is isotopes, if the number of
  neutrons is different the mass will be as well.

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Isotopes

• The reason the masses on the periodic table are
  not whole numbers is also because of isotopes
• As stated before a sample of any given element
  will contain its isotopes as but one of them will
  be more abundant than the others and expressed in
  a percent of all isotopes called abundance
• If you take the abundance, for each isotope,
  and multiply it by the isotopes mass then sum the
  values for all isotopes you will get the weighted
  atomic mass which is what is expressed on the
  periodic table.

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Isotopes

• Lets demonstrate calculation of the weighted
  atomic mass
• Element X has 4 different isotopes what is its
  weighted atomic mass using the data below (this
  is not a real element, remember the abundance
  must always be changed to a decimal when
  multiplying
• Isotope 1, 67.0 abundance, mass 1.03 amu
• Isotope 2, 3.00 abundance, mass 1.23 amu
• Isotope 3, 17.00 abundance, mass 1.09 amu
• Isotope 4, 13 abundance, mass 1.10 amu

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• (.6700)(1.03) (.0300)(1.23) (.1700)(1.09)
  (.1300)(1.03) you figure it out and ask me

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• Isotope X-6 has a mass of 6.015 amu and a percent
  abundance of 7.5, isotope X-7 has a mass of
  92.5, and a abundance of 92.5 what element is
  this on the periodic table using method just
  shown? Again try it on your own then ask me if
  you are correct

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Atomic Models

• Planetary
• Bohr Model
• Which model is correct?

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• As we already know the atom has a centrally
  located nucleus with all the protons and neutrons
  inside the nucleus
• And the atom has mostly empty space with
  electrons orbiting in some place around the
  nucleus at any given time but is the end of the
  description of the atomic model?

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Atomic Model

• How many electrons can actually orbit an atom in
  the same area, or orbit?
• Is there a certain way electrons are placed, or
  found, to be around the atom at any given time?

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Atomic Model

• The next model proposed, after the plum pudding
  model, is called the planetary model, as its name
  suggest you expect to find the electrons orbiting
  the nucleus but all electrons have the same orbit
• This means if I were to take sodium (Na), I would
  draw all the protons and neutrons in the nucleus
  and all 11 electrons in one orbit around the
  nucleus, is this accurate?

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• Instead of saying we have electrons around the
  nucleus in an orbit we can say in an energy level
• Energy levels are where the electrons have the
  highest probability of being found when
  attempting to locate them.

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Atomic Model

• Niels Bohr, a student of Rutherford, proposed his
  own model in 1913 his model was quantized ,which
  you will learn for now very basics, you will
  learn the reasoning behind the Bohr Model in more
  detail later when we study light and quantized
  energy

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Atomic Structure

• Bohr proposed that electrons can only be placed
  in certain energy levels that orbit in certain
  circular orbits around an atom and that each of
  these energy levels can only hold a certain
  number of electrons
• The first energy level is the closest to the
  nucleus, and every energy level added to the atom
  causes the atomic radius to increase

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Atomic Structure

• Now know that the first 5 energy levels can hold
  the following number of electrons 2,8,18,32,50
• To find the number of electrons use the following
  formula 2n2, where n is the number of the energy
  level
• Now take sodium (Na) and draw the Bohr Model for
  an atom.
• You will need 3 energy levels for this one,
  meaning, three orbits around the nucleus each one
  successively larger in diameter going away from
  the nucleus, draw the model now

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• The electrons cannot choose any orbit they wish.
  They are restricted to orbits with only certain
  energies.
• Electrons can jump from one energy level to a
  higher one, or from a higher one to a lower one,
  only when a specific amount of energy is absorbed
  or emitted

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• There is no in between energy levels, the
  electrons can only be in the ground state or an
  excited state.
• For example think of a ladder and the rungs, or
  steps. The steps of the ladder are energy levels
  of an atom and your feet are the electrons. The
  lowest step is the ground state and the higher
  steps are excited states
• Can you stand in between, the rungs, or steps, of
  the ladder,? This is the same concepts are
  electrons and energy levels?

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• That exact, specific, or required, amount of
  energy, enough to place them in a higher energy
  level, or lower energy level is called a quantum,
  plural quanta
• Electrons can be promoted from the lowest energy
  state to a higher energy state as long as they
  have absorbed the same quantum of energy that the
  higher state possesses

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• When an electron goes down from that excited
  state, back to the ground state, it has to give
  off that same amount, quanta, of energy because
  energy can not be destroyed
• When that electron drops back down to a lower
  energy state it gives off that energy as a photon
  which can be seen as light
• The amount of energy that the photon contains is
  just the difference in energy between the excited
  state and the lower state

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Atomic Structure

• Draw the following atoms according to the
  planetary model and the Bohr Model
• Na, Mg, Br, H, He, Ne, Hg, Cu, Fe, Xe, C, Rb