Transcript Historical Model of the Atom

The Historical Development of the
Atomic Model
• From ancient Greece philosophy, to
today’s model of the atom –
• The model of the atom demonstrates the
purpose of science:
“The purpose of science is to create models that explain
natural phenomena.”
Scientific Models
A model is the best possible explanation which
accounts for all observed phenomenon and
has predictability.
1) Best means there can only be ONE model.
2) An unanswered question means change the
model.
3) Predictability is the test of a good or “true”
model.
4) Occham’s Razor – of 2 possible explanations,
choose the simpler.
The ancient Greeks were the first to
postulate that matter consists of
indivisible constituents.
The laws of conservation of mass
and definite proportions provided
the experimental foundation for the
atomic theory
• Law of Conservation of Mass:
– No detectable gain or loss of mass occurs
in chemical reactions. Mass is conserved.
• Law of Definite Proportions:
– In a given chemical compound, the
elements are always combined in the same
proportions by mass.
Dalton’s Atomic Theory:
1. Matter consists of tiny particles called atoms.
2. Atoms are indestructible. In chemical reactions,
the atoms rearrange but they do not themselves
break apart.
3. In any sample of a pure element, all the atoms
are identical in mass and other properties.
4. The atoms of different elements differ in mass
and other properties.
5. In a given compound the constituent atoms are
always present in the same fixed numerical ratio.
Rephrased John Dalton:
•Each element is composed of atoms
•All atoms of an element are identical.
•In chemical reactions, the atoms are not
changed.
•Compounds are formed when atoms of more
than one element combine.
•Dalton’s law of multiple proportions: When two
elements form different compounds, the mass ratio
of the elements in one compound is related to the
mass ratio in the other by a small whole number.
• It follows from Dalton’s Atomic Theory
that atoms of an element have a
constant, characteristic atomic mass or
atomic weight
• For example, for any sample of
hydrogen fluoride:
• F-to-H atom ratio: 1 to 1
• F-to-H mass ratio: 19.0 to 1.00
– This is only possible if each fluorine atom
is 19.0 times heavier than each hydrogen
atom
Later scientists realized that the atom consisted of
charged entities.
•The voltage causes negative particles to move from
the negative electrode to the positive electrode.
*The path of the electrons can be altered by the
presence of a magnetic field.
•Consider cathode rays leaving the positive
electrode through a small hole.
•If they interact with a magnetic field perpendicular
to an applied electric field, the cathode rays can be
deflected by different amounts.
The Discovery of Atomic
Structure
Cathode Rays and Electrons
•The amount of deflection of the cathode
rays depends on the applied magnetic and
electric fields.
•In turn, the amount of deflection also
depends on the charge to mass ratio of the
electron.
•In 1897, Thomson determined the charge to
mass ratio of an electron to be 1.76  108 C/g.
•Goal: find the charge on the electron to
determine its mass.
From the
separation of
radiation we
conclude that the
atom consists of
neutral, positively,
and negatively
charged entities.
Thomson assumed
all these charged
species were found
in a sphere.
Thomson’s Plum-Pudding Model
Goal: find the charge on the electron to determine its
mass.
Consider the following experiment by Robert Millikan
:
•Oil drops are sprayed above a positively charged plate
containing a small hole.
•As the oil drops fall through the hole, they are given a
negative charge.
The Discovery of Atomic
Structure
Cathode Rays and Electrons
Gravity forces the drops
downward. The applied
electric field forces the
drops upward.
When a drop is perfectly
balanced, the weight of the
drop is equal to the
electrostatic force of
attraction between the drop
and the positive plate.
The Discovery of Atomic
Structure
Cathode Rays and Electrons
• Using this experiment, Millikan determined the charge on
the electron to be 1.60  10-19 C.
• Knowing the charge to mass ratio, 1.76  108 C/g,
Millikan calculated the mass of the electron: 9.10  10-28
g.
• With more accurate numbers, we get the mass of the
electron to be 9.10939  10-28 g.
Ernest Rutherford’s Gold Foil
Experiment
• Results of the Rutherford experiment
(a) The results that the metal foil
experiment would have yielded if the
plum pudding model had been correct
(b) Actual results
Rutherford’s Nuclear Model of the atom
•Small, dense, positively charged nucleus
•Contains protons (+1 charge)
•Contains neutrons (no charge)
•Remainder of the atom is mostly empty space
•Contains electrons (-1 charge) in the “empty space”