Download U4: History of the Atom

CHEMISTRY NOTES
Atomic Structure
The Development of the Atomic
Model
•
•
•
•
•
The following men each contributed to
the Atomic model
Democritus
John Dalton
J.J. Thompson
Ernest Rutherford
Neils Bohr
Early Ideas of Matter
•
•
•
Matter could be divided in half again and
again infinitely.
Leucippus (ancient greek philosopher – 400
BC) originated the concept that matter
can only be divided to a point then it
losses its properties.
Leucippus student was Democritus.
Democritus
•
Democritus coined the term
atom
Atom is from Greek
–
•
the prefix "a" means "not" and the
word "tomos" means cut. Our word
atom therefore comes from atomos,
a Greek word meaning uncuttable.
Democritus came about this idea
by way of thought not experiment.
Democritus Main Points
•
•
•
•
•
All matter is composed of atoms, which are
bits of matter too small to be seen. These
atoms CANNOT be further split into smaller
portions.
There is a void, which is empty space between
atoms.
Atoms are completely solid. (WRONG)
Atoms are homogeneous, with no internal
structure. (WRONG)
Atoms are different in size and shape
John Dalton
•
•
•
An English schoolteacher
Father of the present day atomic model
Dalton's idea of an element is what we
believe today –
– an element is a chemical substance that
cannot be decomposed further by chemical
means
•
Lived 1766-1844
Daltons Main Points
•
•
•
•
Chemical elements are made of atoms
The atoms of an element are identical
in their masses (wrong)
Atoms of different elements have
different masses
Atoms only combine in small, whole
number ratios such as 1:1, 1:2, 2:3 and
so on. (Law of Definite Proportions)
– From Proust’s observation
Evidence for The Atomic Theory
Dalton’s Evidence
• Two elements can combine in different ways to
form more that one compound. The masses of
elements in those compounds are still found in
fixed, small whole # ratios
– Law of Multiple Proportions
• Carbon monoxide / carbon dioxide
– Inference  different compounds can be made from
the same elements, just using different amounts
havingsmall, whole-number ratios, again, means there
must be a basic unit of matter (atoms) that combines
to form compounds.
The Law of Definite Proportions
•
•
States: A given chemical compound
always contains the same proportion
by mass of its constituent elements.
Another way to say it: the relative
amount of each element in a particular
compound is always the same,
regardless of preparation or source.
The Law of Definite Proportions
•
•
Water will always have 2 atoms of
hydrogen and 1 atom of oxygen – no
matter how you make it or where it
comes from or how much of it you have.
H 2O
– 2 atoms H X 1 amu = 2 amu
– 1 atoms O X 16 amu = 16 amu
•
•
11.11% of water will always be hydrogen
88.89% of water will always be oxygen
J.J. Thompson
•
•
•
English Physicists
Lived 1856-1940
In 1897, J.J. Thomson discovered the
electron, the first subatomic particle
J.J. Thompson Main Points
•
•
•
Negatively Charges electrons exist
embedded in a positively charged
substance that completely surrounds
them. (WRONG)
The positive material balances out the
negative charges on the electrons so that
the atom is neutral
Under certain conditions electrons could
be removed from the atom.
J.J. Thompson’s Atomic Model
Ernest Rutherford
•
•
•
Professor at Cambridge University
Discovered the nucleus of the atom
Lived 1871-1931
Rutherford’s Experiment
•
•
•
Rutherford worked with alpha particles
Alpha particles are basically a helium
nucleus (i.e. 2 protons and 2 neutrons)
Alpha particles are emitted from
radioactive substances
Rutherford’s Experiment
•
•
Procedure: Rutherford shot alpha
particles at a thin gold foil.
Result: Most of the particles went
straight through the gold foil. However
some of the particles were deflected,
some were even reflected
– According to the Thompson model this
should not happen.
Rutherford’s Experiment
Rutherford’s Experiment
Conclusion:
• Most of the alpha particle going strait
through the gold foil must mean that most
of the atom is empty space.
• According to the Thompson model the
atom should not have enough density or
electrical charge to repel an alpha particle
moving at 10% the speed of light.
Rutherford’s Experiment
Rutherford’s Experiment
•
Rutherford concluded: most of the
mass of an atom and all its positive
charge lie in the very small very dense
region.
– This region was called the nucleus.
– Rutherford named the small, positively
charged particles in the atom protons.
Rutherford’s Experiment
•
Calculations show that the nucleus is
1/100,000th the size of the entire atom,
however over 99% of that atoms mass is
in the nucleus.