Atomic structure

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					Matter is Made up of Atoms

          Chapter 2
Atoms and Their Structure

         Section 2.1

460 BC   Democritus develops the idea of atoms

              He pounded up materials in his mortar

              and pestle until he had reduced them

              to smaller and smaller particles which

              he called

                 (greek for indivisible)

460 BC   Democritus develops the idea of atoms

              The idea that matter is made up of

              fundamental particles called atoms is

              known as the atomic theory of

The well-known
philosophers of the
time, Aristotle and
Plato, had a more
respected, (and
ultimately wrong)

                      Aristotle and Plato favored the earth, fire, air
                      and water approach to the nature of matter.
                      Their ideas held sway because of their
                      eminence as philosophers. The atomos idea
                      was buried for approximately 2000 years.
Development of the Modern Atomic Theory

• In 1782, a French chemist, Antoine Lavoisier
  (1743-1794), made measurements of
  chemical change in a sealed container.

• He observed that the mass of reactants in
  the container before a chemical reaction
  was equal to the mass of the products after
  the reaction.
Development of the Modern Atomic Theory

• Lavoisier concluded that when a chemical
  reaction occurs, mass is neither created nor
  destroyed but only changed.
• Lavoisier’s
  became known as
  the law of
  conservation of
Development of the Modern Atomic Theory

• In 1799, another French chemist, Joseph
  Proust, observed that the composition of
  water is always 11 percent hydrogen and 89
  percent oxygen by mass.
• Regardless of the
  source of the water, it
  always contains these
  same percentages of
  hydrogen and oxygen.
Development of the Modern Atomic Theory

• Proust studied many other compounds and
  observed that the elements that composed
  the compounds were always in a certain
  proportion by mass. This principle is now
  referred to as the law of definite proportions
         Dalton’s Atomic Theory

• John Dalton (1766-
  1844), an English
  schoolteacher and
  chemist, studied the
  results of
  experiments by
  Lavoisier, Proust,
  and many other
          Dalton’s Atomic Theory
The following statements are the main points of
 Dalton’s atomic theory.

1. All matter is made up of atoms.
2. Atoms are indestructible and cannot be divided
into smaller particles (Atoms are indivisible).
3. All atoms of one element are exactly alike, but
are different from atoms of other elements.
Dalton’s Atomic Theory

   • Proposed his atomic theory of
     matter in 1803.
   • Although his theory has been
     modified slightly to accommodate
     new discoveries, Dalton’s theory
     was so insightful that it has
     remained essentially intact up to
     the present time.

1897   Joseph John Thompson

           British physicist, J.J. Thomson,
           discovered that Dalton’s solid-ball
           model was not accurate.

           Thomson’s experiments used a vacuum
    Cathode-Ray Tube

• A vacuum tube has
  had all gases pumped
  out of it.
• At each end of the tube is a metal piece
  called an electrode, which is connected
  through the glass to a metal terminal outside
  the tube.
• These electrodes become electrically charged
  when they are connected to a high-voltage
  electrical source.
    Cathode-Ray Tube
• When the
  electrodes are
  charged, rays
  travel in the tube
  from the negative
  electrode, which     • Because these
  is the cathode, to     rays originate at
  the positive           the cathode, they
  electrode, the         are called
  anode.                 cathode rays.
      Cathode-Ray Tube
• Thomson found that the rays bent toward a
  positively charged plate and away from a
  negatively charged plate.

• Thomson concluded
  that cathode rays
  are made up of
  negatively charged
  particles referred
  to as electrons.

His idea was that an atom was made up of electrons

scattered unevenly within an elastic sphere

surrounded by a soup of positive charge to balance the

electron's charge like plums surrounded by pudding.

                           PLUM PUDDING
      Cathode-Ray Tube
• From Thomson’s experiments, scientists had
  to conclude that atoms were not just neutral
  spheres, but somehow were composed of
  electrically charged particles.
• If atoms contained extremely light,
  negatively charged particles, then they must
  also contain positively charged particles.
• In 1886, scientists discovered that a
  cathode-ray tube emitted rays not only from
  the cathode but also from the positively
  charged anode.
• Thomson was able to show that these rays
  had a positive electrical charge.
• Years later, scientists determined that the
  rays were composed of positively charged
  subatomic particles called protons.
• At this point, it seemed that atoms were
  made up of equal numbers of protons and
• However, in 1910, Thomson discovered that
  neon consisted
 of atoms of
 two different
• Atoms of an element that are chemically
  alike but differ in mass are called isotopes
  of the element.
• Today, chemists know that neon consists of
  three naturally occurring isotopes.
• Because of the discovery of isotopes,
  scientists hypothesized that atoms
  contained still a third type of particle that
  explained these differences in mass.
• The existence of this neutral particle, called
  a neutron, was confirmed in the early 1930s.
     Rutherford’s Gold Foil Experiment
• In 1909, a team of
  scientists led by Ernest
  Rutherford in England
  carried out the first of
  several important
  experiments that
  revealed an arrangement
  far different from the
  cookie-dough model of
  the atom.
    Rutherford’s Gold Foil Experiment
Conducted an experiment
 to see if alpha particles
 would be deflected as
 they passed through a
 thin foil of gold.

Rutherford was aware of
 Thomson’s “plum pudding”
 model and expected only
 minor deflections of
 alpha particles.
            Rutherford’s Model
• To explain the results of the experiment,
  Rutherford’s team proposed a new model
  of the atom.

• Because most of
  the particles
  passed through
  the foil, they
  concluded that the
  atom is nearly all
  empty space.
               Rutherford’s Model

He suggested that
the positive charge
was all in the central
nucleus. With this
holding the electrons
in place by electrical

  However, this was not the end of the story…
1913   Niels Bohr
              Studied under Rutherford at the Victoria
              University in Manchester.

              Bohr refined Rutherford's idea by
              adding that the electrons were in
              orbits. Rather like planets orbiting the
              sun. With each orbit only able to
              contain a set number of electrons.


electrons in orbits

Particle    Charge     Mass

proton     + charge     1

neutron    No charge    1

electron   - charge     0
              Atomic Numbers
• The atomic number of an element is the
  number of protons in the nucleus of an atom
  of that element.
• It is the number of
  protons that
  determines the
  identity of an
  element, as well as
  many of its chemical
  and physical
               Atomic Numbers
• Because atoms have no overall electrical
  charge, an atom must have as many electrons
  as there are protons in its nucleus.
• Therefore, the
  atomic number of an
  element also tells the
  number of electrons
  in a neutral atom of
  that element.
• The mass of a neutron is almost the same as
  the mass of a proton.
• The sum of the protons and neutrons in the
  nucleus is the mass number of that
  particular atom.
• Isotopes of an element have different mass
  numbers because they have different
  numbers of neutrons, but they all have the
  same atomic number.
                 Atomic Mass
• In order to have a simpler way of comparing
  the masses of individual atoms, chemists
  have devised a different unit of mass called
  an atomic mass unit, which is given the
  symbol u.

• An atom of the carbon-12 isotope contains
  six protons and six neutrons and has a mass
  number of 12.
                 Atomic Mass
• Chemists have defined the carbon-12 atom
  as having a mass of 12 atomic mass units.

• Therefore, 1 u = 1/12 the mass of a carbon-
  12 atom.

• 1 u is approximately the mass of a single
  proton or neutron.
   Information in the Periodic Table
• The number
  at the bottom
  of each box is
  the average
  atomic mass
  of that

• This number is the weighted average mass
  of all the naturally occurring isotopes of
  that element.
     Atomic   Mass   p+   n0   e–

Ca    20       40    20   20   20

Ar    18       40    18   22   18

Br    35       80    35   45   35

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