Transcript Atomic structure (download)

Atoms are not the
smallest thing
Growing evidence for the divisibility
of the indivisible
Electrostatics and electricity
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The phenomenon of static electricity was
known since ancient times. Certain “charged”
objects repel and others attract
The discovery of the voltaic cell allowed the
harnessing of electrical current from chemical
activity
Electrical generation was achieved in 1825
Atoms are neutral. If indivisible, with what
are the electrical charges associated?
Faraday’s prescience
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“Although we know nothing of what an atom
is, we cannot resist forming some idea of a
small particle; and though we are in equal
ignorance of electricity, there is an immensity
of facts which justify us in believing that the
atoms of matter are associated with electrical
powers to which they owe their most striking
qualities, and amongst them their chemical
affinity.”
Ray of hope
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1858
Discovery of cathode rays by Julius Plucker
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Application of a large voltage across an evacuated
tube causes a current to flow. The current flow is
accompanied by radiation from the excited gas
molecules
How does the neutral and indivisible atom create a
charge?
Cathode rays are negatively
charged particles
1897.J. Thomson demonstrates that cathode rays
consist of negatively charged particles, which have a mass
very much less than that of the atom. The first sighting of
the electron.
The “oil drop” experiment
gave a measurement of
electron charge
The Thomson model of the atom
"I regard the atom as containing a large number of smaller bodies
which I will call corpuscles, these corpuscles are equal to each
other.... In the normal atom, this assemblage of corpuscles
forms a system which is electrically neutral. Though the
individual corpuscles behave
like negative ions, yet when they are
assembled in a neutral atom the negative
effect is balanced by something which
causes the space through which the corpuscles are spread to act as
if it had a charge of positive electricity equal in amount to the
sum of the negative charges of the corpuscles…”
A hierarchy of discovery
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Investigations of cathode rays led to the
discovery of X-rays in 1895 (Wilhelm
Roentgen)
This new discovery inspired search for other
types of invisible emission by substances
The world would never be the same again…
Radioactivity
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1896
Henri Becquerel discovered radioactivity,
which suggested that some atoms were capable
of decomposing to give smaller particles.
1903
Frederick Soddy and William Ramsey
demonstrated that uranium decayed to give
helium. Direct proof that atoms were
divisible.
1909 Ernest Rutherford’s
gold foil experiment with
alpha particles and the
discovery of the nucleus.
The nucleus
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Tiny
Incredibly dense – contains all the mass
of the atom
Positively charged
Contains protons (charged) and neutrons
(neutral) – not discovered until much
later
Summary of Atom Pictures
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Dalton: Indivisible atom
Thomson: Electrons
Rutherford: Nucleus
Atoms are mostly nothing
Comparison of subatomic particles
Mass
Charge
Particle
grams
amu
coulombs
5.5x10-4 -1.6x10-19
e
Electron
9.1x10-28
Proton
1.67x10-24 1.007
+1.6x10-19 +1
Neutron
1.67x10-24 1.009
0
-1
0
Atoms are neutral:
# electrons = # protons
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The number of protons in the nucleus of an atom
determines the atomic number (Z) and indicates the
element's identity. For a neutral atom, the atomic
number also describes the number of electrons around
the nucleus.
Variations on the number of
neutrons in the nucleus give rise to
different isotopes of the same
element.
Element notation: Atomic number
and mass number
Mass
number –
protons +
neutrons
Atomi
number –
protons
12
6
C
Element
symbol
Isotopes and relative atomic masses
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It is important to be able to calculate the
average mass of a mixture of isotopes of a
given element
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We need the distribution of the isotopes and their
mass numbers
For chlorine there are isotopes with mass numbers
35 and 37, with relative abundances of 75.8 % and
24.2 % respectively
Average atomic mass =
(35 x0.758)  (37 x0.242)  35.5
What of the electrons?
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We now understand the atom to contain a tiny
positively charged massive nucleus surrounded
by a comparatively vast empty space
containing the electrons
When atoms combine the electrons must
interact
We need to understand the arrangement of
electrons in the atom