Atoms_ by chenmeixiu

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									Atomic Structure and
   Development
Honors and Pre-AP Chemistry
          Today’s elements
 Let’s start out with the building blocks of
  your body:
 Carbon, C
 Hydrogen, H
 Oxygen, O
 Nitrogen, N
 Phosphorous, P
 Sulfur, S
 Remember this as CHONPS.
Atoms and their building blocks.
 ATOMS are the smallest units of matter
  which contain the chemical properties that
  make an element unique.
 All of the elements, therefore, are made up
  of different types of atoms. In order to
  understand how this can be, we must look
  at what makes up atoms.
   Atoms are made up of two main parts: the
    NUCLEUS and the ELECTRON CLOUD.
   The NUCLEUS is a tiny sphere at the core
    of an atom containing two different types
    of particles, NEUTRONS (0 charge) and
    PROTONS (+1 charge).
   The ELECTRON CLOUD is a massive
    area around the nucleus full of particles
    traveling very rapidly known as
    ELECTRONS (-1 charge).
   The tiny core of the atom with its positive charge
    holds onto the electrons, which are negative,
    causing them to spin around the nucleus at
    incredible speeds.
   This is due to the fact that opposite charges
    attract, known as THE LAW OF
    ELECTROSTATIC ATTRACTION.
           Subatomic Particles
   Protons
        Mass 1
        Charge +1

   Neutrons
        Mass 1
        Charge 0

   Electrons
        Mass ~0
        Charge -1
     Reading the Periodic Table
   Each symbol on the Table allows you to
    determine the number of protons,
    neutrons, and electrons in an element.
               Atomic Numbers
   The atomic number at the TOP of each
    symbol gives the number of PROTONS.
       This number is unique for each element and
        determines its identity.
               Atomic Mass
   The BOTTOM number gives the weighted
    average mass of an element’s atoms.
     Combined mass of PROTONS and
      NEUTRONS. Electrons don’t weigh enough
      to really count here.
     This number is NOT a whole number due to
      the fact that an element may exist in many
      versions known as ISOTOPES.
                A note on Isotopes

 Isotopes are atoms of the same element
  that have different masses.
 Isotopes of an element share the same
  number of protons, but will have different
  numbers of neutrons.
 Isotopes will have different masses than
  those shown on the Periodic Table and
  are written “Name – Mass”
       Ex. Uranium-238
                   Finding Protons


   Unique to each element, listed at the top
    of each symbol on the periodic table.
       Ex. Carbon = 6, Oxygen = 8 …..
   Will be the same for all isotopes of the
    same element.
               Finding Electrons


 For neutral atoms (not ions), the number
  of electrons will equal the number of
  protons.
 Cations (+ atoms) have fewer electrons
 Anions (- atoms) have more electrons
                A note on Ions
 Neutral atoms have equal electrons (-) and
  protons (+).
 An atom becomes an ion when it gains or
  loses electrons.
 Lose electrons, become (+) and cation.
       Ex. Cu  Cu1+  Cu2+
   Gain electrons, become (-) and anion.
       Ex. O  O1-  O2-
                 Finding Neutrons

   To find neutrons, subtract the atomic
    number (protons) from the atomic mass
    (neutrons + protons).
       (Mass) – (Atomic #) = Neutrons
 To keep things simple, round the mass to
  the nearest whole number.
 This is the only number that changes in
  isotopes.
     History and Development
 The structure of the atom is the result of
  research by many scientists spanning
  several thousand years.
 What follows is a story over 2500 years.
             The Philosophers
   Curiosity is what lead scholarly thinkers, also
    known as philosophers, to consider the
    mysteries of life.
   They thought about the nature of matter and
    made explanations based on their own life
    experiences.
   They thought that matter was made of things like
    earth, water, air and fire.
   There was no way, however, to test the validity
    of their ideas.
                  Democritus

   The Greek philosopher Democritus (460-370
    BC) was the first to propose that matter was not
    infinitely divisible.
   He believed that matter was made up of tiny
    particles he called atomos (where we get our
    word atom.)
   He believed that atoms could not be created,
    destroyed, or further divided.
   He was really ahead of his time, and received a
    lot of criticism for his theory.
             Democritus’ Ideas
   Matter is composed of empty space through
    which atoms move.
   Atoms are solid, homogeneous, indestructible
    and indivisible.
   Different kinds of atoms have different sizes and
    shapes.
   The differing properties of matter are due to the
    size, shape, and movement of atoms.
   Apparent changes in matter result from changes
    in the groupings of atoms and not from changes
    in the atoms themselves.
                     Aristotle
   The Greek philosopher Aristotle (384-322 BC)
    rejected Democritus’s ideas because it did not
    agree with his ideas on nature.
   He criticized the idea that atoms moved through
    empty space; he did not believe that empty
    space existed.
   Eventually Democritus’s theory was rejected
    because he was not able to answer the
    questions about his theory.
   Unfortunately, Aristotle gained wide support for
    his ideas on nature and his ideas went
    unchallenged for about 2000 years.
Aristotle describes the 4 elements…
   Describes the 4 elements
       "...four bodies are fire, air, water, earth." (339a15-16)
       "Fire occupies the highest place among them all, earth
        the lowest, and two elements correspond to these in their
        relation to one another, air being nearest to fire, water to
        earth." (339a16-19)
       "Fire, air, water, earth, we assert, originate from one
        another, and each of them exists potentially in each, as
        all things do that can be resolved into a common and
        ultimate substrate." (339a36-b2)
   As translated, Aristotle refers to bodies as
    elementals. All earthly substances originate from
    these four elements. When comparing a tree to a
    rock, different amounts of the four elementals exist
    in each to create the differences we see or
    observe.
       Intermission

   (Skip ahead nearly 2000
years…..and yes, Aristotle really
     did screw things up.)
             John Dalton
 The concept of the atom resurfaced in the
  18th century, but took another 100 years
  before any progress was made.
 John Dalton (1766-1844) was a school
  teacher in England and he revived and
  revised Democritus’s ideas based on the
  results of scientific research that he
  conducted.
         Dalton’s Atomic Theory
   (TRUE) All matter is composed of extremely small
    particles called atoms.
   (Mostly TRUE) All atoms of a given element are identical
    having the same size, mass, and chemical properties.
    Atoms of a specific element are different from those of
    any other element.
   (FALSE!) Atoms cannot be created, divided into smaller
    particles, or destroyed.
   (TRUE) Different atoms combine in simple whole-
    number ratios to form compounds.
   (TRUE) In a chemical reaction, atoms are separated,
    combined, or rearranged.
   Solid Sphere Model or
    Billiard Ball Model
    proposed by John Dalton
             Finally…Progress!
   Dalton studied many chemical reactions and
    made careful observations and measurements
    along the way.
   He was able to accurately determine the mass
    ratios of the elements involved in the reactions.
    Based on that information, he proposed his
    atomic theory in 1803.
   Dalton’s convincing experimental evidence and
    clear explanation of the composition of
    compounds and conservation of mass led to the
    general acceptance of his atomic theory.
    Problems with Dalton’s Theory

   Dalton was wrong about a few things:
     Atoms can be divided into their subatomic
      particles.
     All atoms of a given element do not
      necessarily have identical
      properties…Isotopes!
     Discovery of the Electron
 Observations related to static electricity
  led scientists in the 1800’s to look for a
  relationship between matter and electric
  charge.
 They used the newly discovered vacuum
  tube to pass electricity through a glass
  tube that had most of the air (matter)
  removed.
 Sir William Crookes noticed a flash
  of light inside one of the tubes.
  The flash was made by some kind
  of radiation striking a light-
  producing coating on the end of
  the tube.
 Crookes cathode
  ray deflecting tube.
 More work showed that there were rays
  moving from the cathode (negative end) to the
  anode (positive end) which became known as
  a cathode ray.
 The cathode ray led to the invention of the
  television and computer monitor; these make
  the images when radiation from the cathode
  strikes light-producing chemicals coating the
  back side of the screen.
                             The first RCA Color TV tube
                             1947
                             This was the first working
                             shadowmask Delta color TV
                             tube with a small screen.
         The progress so far…
   By the end of the 1800’s scientists were
    relatively sure about the following:
     Cathode rays were actually a stream of
      charged particles (ions!)
     The particles carried a negative charge
    **Because the ray was not affected by changing
      the gas or type of electrode, they concluded
      that the negatively charged particles are part
      of all forms of matter…ELECTRONS!
                J.J. Thomson

   No one had yet figured out the mass of a single
    cathode ray particle.
   English physicist J.J. Thomson did a series of
    cathode ray tube experiments in the late 1800’s
    to determine the ratio of its charge to its mass.
   Thomson concluded that the mass of the particle
    was less than the hydrogen atom ( which was
    the lightest known atom).
   This meant that Dalton was wrong. Thomson
    had found the first subatomic particle… the
    electron!
Thomson’s original gas tube used to discover
               the electron.
                      Robert Millikan
   In 1909, American physicist Robert Millikan
    determined the charge of an electron.
   His oil drop experiment and equipment was
    done so well that his measured value is within
    1% of the currently accepted value!
   He determined that a single electron has a
    charge of 1-. Then knowing the charge to mass
    ratio from Thomson, he calculated the mass of a
    single electron:
          9.1 X 10 -28 g
   The atomizer produced a fine spray of oil droplets with a radius of
    about 1 µm. Many of the droplets were charged. Occasionally a
    droplet fell through the pinhole. If the droplet was charged, it could
    be brought to a halt and held stationary by applying a voltage
    across the metal plates.
   When the droplet was stationary, the force exerted by the
    electrostatic field, EQ (where E is the field and Q is the charge on
    the droplet), was equal to the weight of the droplet, mg, where m is
    its mass and g the acceleration due to gravity.
   E can be calculated by measuring the voltage across the plates.
       The Plum Pudding Model
   With the new information and to answer
    questions about the neutrality of matter, J.J.
    Thomson proposed a new model of the atom.
   The Plum Pudding model consisted of a
    spherically shaped atom consisting of a
    uniformly distributed positive charge with
    negatively charged electrons dispersed
    throughout.
   A more modern name would possibly be the
    chocolate chip cookie model.
   Pudding Model or
    Raisin Bun Model
    proposed by J.J.
    Thomson
             Ernest Rutherford
   In 1911, Ernest Rutherford, an English scientist,
    became interested in studying how positively
    charged alpha particles (a kind of radiation)
    interacted with solid matter.
   He and some colleagues made an experiment
    where he passed alpha particles through gold
    foil and measured how often and in what
    direction alpha particles bounced off the foil.
   He looked at the angles that the particles
    bounced off the foil and concluded that the plum
    pudding model must be incorrect.
Figure 1B. Rutherford Gold Foil Experiment Apparatus in a cloud
chamber




                                                                  Figure 1C. Alpha particles interacting with a gold atom in the gold foil
        Rutherford’s Conclusion
   He calculated from his experiment results that
    the atom must consist mostly of empty space
    through which the electrons move.
   There was a tiny, dense region, which he called
    the nucleus, in the center of the atom that
    contained all the atom’s positive charge and
    virtually all its mass.
   The electrons, then, move around in empty
    space and are held in orbit by the attraction for
    the positively charged nucleus.
   His model is called the Nuclear Model.
    What about protons and neutrons?
 By 1920, Rutherford had refined his concept
  of the nucleus. He decided that the nucleus
  contains positively charged particles he
  called protons.
 In 1932, Rutherford’s co-worker James
  Chadwick showed that the nucleus also
  contained another subatomic particle, the
  neutron.
    People and Ideas to Know
 Democritus (“Atomos”)
 Aristotle (4 elements)
 John Dalton (Many good ideas)
 J.J. Thomson (Electrons)
 Robert Millikan (More on electrons)
 Ernest Rutherford (Nucleus/protons)
 James Chadwick (Neutrons)

								
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