Download STRUCTURE_OF_THE_ATOM

A Short History
 The
Greeks & the Hindus appear to have
developed theories of matter
 The Greeks get more credit because more of
their writings have survived.
 Empedocles
of Agrigentum is credited with
the concept of the 4 elements:
earth, air, fire and water
 First
 All
Atomic Theory
material things
consisted of small,
uncuttable particles
that only differed in
size, shape, position
and mass, called
atomon
 Didn’t
agree with
Democritus.
 Added the properties
of matter,
moist, dry, hot & cold
to the 4 elements
The Greeks weren’t big
on experimentation.
These were mostly just
philosophical ideas
 Not
much changed for the next 1000 years.
The ideas of Aristotle were still being taught.
 However
starting in the 1500s, atomism and
the idea of “corpuscles” became more talked
about.
 Gassendi,
Boyle & Newton all had ideas of
what an atom was.
 Not
a chemist at first.
 First studied meteorology.
 Was colour blind and wrote a paper
describing it.
 In 1800 started experimenting with
gases and vapours.
 His numerous, well-recorded
experiments led to the New System
of Chemical Philosophy in 1808.
Elements are made of extremely small particles
called atoms.
 Atoms of a given element are identical in size,
mass, and other properties; atoms of different
elements differ in size, mass, and other
properties.
 Atoms cannot be subdivided, created, or
destroyed.
 Atoms of different elements combine in simple
whole-number ratios to form chemical
compounds
 In chemical reactions, atoms are combined,
separated, or rearranged.

Dalton used his
own symbols to
represent the
structures of
compounds.
 Law
of Definite Proportions (or Constant
Composition)

Samples of a compound always contain the same
proportion by mass of the elements.
 Law

of Multiple Proportions
Elements combine to form compounds in whole
number ratios
 Law

of Conservation of Mass
Mass of reactants = mass of products
 Lots
of chemistry and physics research was
being done but not a lot of direct evidence
for atoms.
 In fact the subatomic particles (the smaller
bits of the atoms) were hypothesized first.
 Did
experiments with
cathode ray tubes.
 Confirmed presence of
negatively charged
particles.
 Hypothesized
presence of positive
charge as well.
e/m = -1.759 x 108 coulomb/gram - 1897
Video: Discovery of the
electron
 Thomson
guessed that the electrons were
like raisins scattered throughout a spherical
ball of positive charge – much like an English
pudding.
 Gold
Foil Experiment -
1911
 Worked with
radioactive particles
 Discovered the



Alpha
Beta
Gamma particles
 Discovered
atomic
nucleus
 Predicted existence of
neutron
 Got
2 of his students (Geiger & Marsden) to
experiment with -particles.
 They passed -particles through a thin sheet
of gold foil.
Video: Long Ruthers Alpha Scattering

Gold foil experiment showed

Most particles went straight through


Mostly empty space
A few bounced back or were deflected at an angle.

Tiny region of concentrated mass – the nucleus.
Eventually found to have + charge = to the atomic
number
 Protons contained most of the mass.


Video: Discovery of the Nucleus
 Electron

Cloud Model
Tiny +ve nucleus surrounded by cloud of
negative electrons.
 1913
– Tried to
explain spectral lines.
 Proposed that
electrons are
restricted to “orbits”
at a fixed distance
from nucleus and
could move from 1
orbit to another.
 Would give off or
absorb energy as they
jumped.
 Planetary
Model

Used Rutherford’s
basic model but
added the orbits.

Model has evolved
well beyond this, but
this model is still
useful for explaining
some ideas related to
the atom.
 Chadwick,
a student of Rutherford and
worked with Geiger
 Discovered the neutron in 1932.
 Many
subatomic particles have since been
discovered. Particle physics & quantum
mechanics examine how these subatomic
particles behave.
Video: The existence of quarks