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Transcript
Models of the Atom
The Nucleus
Early Beliefs
 2500 ya, early Greek philosophers believed
that if you continued to divide matter
eventually you would have only one particle
left they called atom (means cannot be
divided)
Models of the Atom
 In the 18th Century man began to study why
some substances could go together and how
some could be taken apart
 Scientists called substances which couldn’t
be broken down into simpler substances as
elements
 Elements were pure and made of only one
kind of atoms
 Silver, gold, iron, carbon and oxygen are
types of elements (90 natural occuring
elements)
Dalton’s Concept
 Early 19th century John Dalton proposed ideas
about matter
 1. Matter is
made up of atoms
 2. Atoms cannot be divided into smaller pieces
 3. All the atoms of an element are exactly alike
 4. Different elements are made of different kinds
of atoms
 Proposed that an atom looked like a tiny hard
marble that was the same throughout
Crookes Cathode Experiment
 Late 19th Century William Crooke took a vacumn tube,
attached battery to two electrodes (anode-positive
charge, and cathode-negative charge)
 Placed a cross in middle a of tube
 Connected the electric current and a shadow was
shown on far end of tube (anode)
 What was the beam? Crooke thought it was a beam of
charged particles… called cathode rays
Crookes Cathode Tube
Discovering Charged Particles
 JJ Thomson placed magnet next to cathode tube and
bent the green glow light (light can’t be bent, so it
must have been charged particles)
 Thomson concluded they were negative charged
particles, now charged electrons
Thomson’s Cathode Tube
Thomson’s Atomic Model
 Atoms are neutral, but if there were negative charged
electrons there must also be positive charged particles
 There would have to be positive charges to balance the
negative charges
 Thomson’s model of an Atom
 A sphere of positive and negative charged particles
evenly spaced kind of like cookie dough and chocolate
chip
Rutherford’s Experiment
 Tested cookie dough theory
 Fired alpha particles through thin gold foil
 If positively charged particles are evenly spaced in
atom, the alpha beam will pass through without
deflection
 Found some deflection which indicated the particles
would have to be concentrated in atom not randomly
scattered
 Proposed a new shape of atom with a concentrated
nucleus of positive charged particles
 1920 proposed the positive charge particles as protons
Rutherford’s Experiment
Atomic Model Changed again
 The nucleus has two much mass for just positive
protons
 Proposed that there is particles the same size as
protons but have no charge in the nucleus
 Called these new particles neutrons
 Now the atomic model is a tightly spaced nucleus
composed of protons(+) and neutrons (no charge)
surrounded by a cloud of much light weighted
electrons (-)
The Nucleus
 An element is defined by the number of protons
present in nucleus. (another name for number of
protons is atomic number)
 Neutrons however can have varying numbers of
neutrons in nucleus
 When atoms of the same element varying in number
of neutrons it is called an isotope
 Carbon normally has 6 proton and 6 neutrons, but
carbon atoms can sometimes have 7 or 8 neutrons in
the nucleus, thus those are isotopes of Carbon

Number of Neutrons
 Carbon normally has 6 proton and 6 neutrons, but
carbon atoms can sometimes have 6, 7 or 8 neutrons in
the nucleus, thus those are isotopes of Carbon
 Isotopes of Carbon’s mass is written as totally the
numbers of protons and neutrons:
 Carbon 12 (6 protons and 6 neutrons)
 Carbon 13 (6 protons and 7 neutrons)
 Carbon 14 (6 protons and 8 neutrons)
Mass number
 Mass number of an isotope is the number
of neutrons plus protons
 To find the number of neutrons in an
isotope subtract the atomic number from
the mass number
 Carbon 14 means it has a mass of 14
 Carbon has 6 protons thus an atomic number of
6
 14(mass#) – 6 (protons) = 8 (neutrons)
Strong Nuclear Force
 Because the nucleus is composed of positive charged
protons, you would think that the like charges would
repel each other
 Rather strong nuclear forces hold the protons
together only when they are closely packed as they are
in the nucleus of the atom
Radioactive Decay
 Nucleus are stable when the number of protons and
neutrons are similar (ie: 6 protons, 6 neutrons)
 Some nuclei are unstable because they have too many
or too few neutrons
 These unstable nuclei release particles to become
more stable
 The release of nuclear particles and energy is called
radioactive decay
 When particles like protons are emitted from nucleus,
the Atomic # changes and a new atom is formed
 Transmutation is changing of one element into
another through radioactive decay
Loss of Alpha Particle
 When an alpha particle is emitted from a radioactive
element 2 protons and 2 neutrons are lost from
nucleus (atomic mass of 4)
 The resulting atom has 2 less protons and atomic mass
is 4 less
 U-238 releases alpha particle (α) and Th-234 forms


238
92U
234
90
4
Th
 New element forms
+
α
2
Loss of Beta particles
 Some unstable elements undergo transmutation where
a neutron becomes unstable and splits into an electron
and a proton and a beta particle (β) is released
 The electron is emitted as high energy and the proton
remains in the nucleus increasing the atomic number
by one and changing the element
 Atomic number therefore is increased by one but the
atomic mass stays the same because a neutron changes
to a proton (same mass)


234
90
234
Th
91
0
Pa + -1 β ↝
Rate of Decay
 Radioactive decay is random
 Rate of decay of a nucleus is measured by its half-life
 Half-life of radioactive isotope is the amount of time
it takes for half of a sample of the element to decay
 For every half-life, ½ of the original mass is gone and
you can calculate how much will be left after each half
life
Half life table
Number of
half-lives
elapsed
Fraction
remaining
0
1/
1
100
1
1/
2
50
2
1/
4
25
3
1/
8
12
.5
4
1/
16
6
.25
5
1/
32
3
.125
6
1/
64
1
.563
7
1/
128
0
.781
...
...
Percentage
remaining
...
Carbon Dating
 Half life of Carbon is 5730 years
 C14 is taken in by plants just as C12 is and when
organism dies radioactive decay continues and can be
calculated into age
 We can calculate age of fossils by calculating how
much C14 (radioactive) remains in a sample (accuracy
to 35000 years)
 Uranium also is used to date rocks, however its half life
is 4.5 billions years and decays to lead (Pb) scientists
calculate age of earth and rocks by comparing amount
uranium remaining and lead (Pb) formed
Making Synthetic Elements
 Synthetic elements are man made elements caused
by smashing elements with alpha and beta particles
 These new elements are not found in nature
 Atomic numbers 93 to 112 and 114 have been made this
way
Uses of Radioactive Isotopes
 Tracer elements: radioactive isotopes that are
introduced into an organism where it is used to
diagnose disease and study its surrounding
 Isotopes have short half-lifes
 Other radioactive elements are introduced to detect
cancer, digestion problems and circulation problems
 Tumors and fractures can be found using isotopes
 Radioactive isotopes can be used to trace phosphates
in plants, trace pesticides as it moves through
ecosystems