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Development of the
atomic theory
Important laws
• Law of conservation of mass
– Mass is neither created or destroyed during
ordinary chemical reactions or physical changes.
• All the matter that has ever existed still exists and
always will
• It can change form or location but can always be
accounted for.
Important laws
• Law of definite proportions
• A chemical compound always contains the
same elements in the same proportions by
mass.
• Law of multiple proportions
– If two or more different compounds are formed
from the same elements then the masses of the
second element when the first remains constant is
always a small whole number.
Dalton’s Atomic Theory
• All matter is composed 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
Dalton’s Atomic Theory continued
• Atoms of different elements combine in
simple whole-number ratios to form chemical
compounds
• In chemical reactions, atoms are combined,
separated or rearranged.
First modification to Dalton’s Theory
Mid to late 1800’s
Major advancement in society was….
Use of electricity
Scientists had a new toy
JJ Thomson 1897
Thomson’s findings:
•A green beam travelled from the
Cathode to the anode.
•There was nothing in the tube except
the metal plates that made the
electrodes
•The beam had a mass and a negative
charge.
Conclusions:
• The beam is made of particles
From the cathode
•The particles,called electrons,
have a negative charge
•0.0065g of electrons have 6.02x1023
negative charges
•The negative charges come from the
atoms in the metal plate
•Atoms can be broken into smaller
particles.
Now that we know the atom has parts………..
Discovery of them begins.
atoms are neutral
atoms have negtive charged particles
therefore there must also be….
positive parts that balance out the
negative.
What might that look like?
First attempt: JJ Thomson
The model failed…………
There was no experiment that confirmed it.
There was an experiment that contradicted it.
Along came Rutherford…
• New Zealander
•Hired to head a team to study the
atom.
• Extremely well known for his
discoveries
Although it was carried out by
Geiger and Marsden
Observations
• Most positive particles went straight through the gold foil
• Some deflected to the side after passing through the gold foil
• A few never made it through and bounced straight back.
Conclusions:
• A large positive force deflects the
positive particles
• Most of the particles pass through
empty space.
• Most of the mass is in the dense
positive center.
The Model
The model description
• The atom has a dense positive core called
the nucleus
• The mass of the atom is in the nucleus
• The electrons move around the nucleus
• The atom is mostly empty space.
Discovery of the proton
By 1920 Rutherford’s team discovered that:
•The hydrogen atom contained 1 positive charge
•The hydrogen nucleus contained one particle
• Each positive particle contained 1 positive charge
•Rutherford called it a proton from the Greek word for first
Neutron:
•All of the mass of an atom could not be accounted for by
the electrons and protons.
•Rutherford determined that there must be a neutral
particle that was still undiscovered.
•In 1932 James Chadwick published a report that he had
found the neutron. It’s existence was confirmed a year later
by Booth.
NEXT
QUESTION?
HOW DO
THE
ELECTRONS
MOVE AROUND THE
NUCLEUS?
The Bohr Atom Model