Our current class model of the atom
John Dalton’s Model
element
Element/molecule
Molecule/compound
All matter is made of atoms
• Smallest form of matter
• Atoms are believed to be indivisible (nothing smaller
than an atom)
“Billiard ball” model
Cathode Ray Tube Experiment
JJ Thomson (1897)
Cathode: negatively charged electrode (attracts positively-charged
things)
Anode: Positively charged electrode (attracts negatively-charged
things)
cathode
anode
Majority of gas (air)
inside tube is pumped
out
Cathode Ray Tube Experiment
The identity of this
cathode ray was a
topic of
research/discussion for
more than 40 years
• No one knew what
it was!
Many ideas emerged, but explanation began with the
question of whether or not this cathode ray had an electric
charge
• In order to find out, it seemed reasonable to place it
within an electric field
Electric plates are charged
• Cathode ray is attracted to
positive electrode
• This can only mean that the
cathode ray is negatively charged
Thomson wanted to investigate the idea of the cathode ray having a
charge even further by placing it in a magnetic field
• He knows that if something has a negative electric charge and its
moving, a magnet will cause it to bend a certain way
This provided further proof that whatever the
cathode ray was composed of was most likely
negatively charged
A few conclusions can be made…
1.) Cathode rays
must be made of stuff that is negatively
charged
• Was attracted to positively-charged anode
• Cathode ray bent in the direction Thomson predicted it would in the
presence of a magnetic field
• Cathode ray bent in the direction toward positively-charged plate
2.) Particles
that make up cathode rays are over 1,000
times smaller than a hydrogen atom
• Was able to determine the ratio of mass to charge and compare that to a
hydrogen atom (smallest bit of matter known at the time)
3.) All different metals give off cathode rays
• Regardless of switching out different metals for the anode and cathode,
Thomson found that the size of the stuff that made up the cathode rays
was the same every time
To sum it up….
1.) Cathode rays must be made of stuff that is negatively charged
2.) Particles that make up cathode rays are over 1,000 times smaller than the
hydrogen atom
3.) All different metals give off cathode rays
Atoms have tiny, negatively
charged particles inside them
Electrons!!
Time for a new model
John Dalton’s Model
Billiard ball model
• Smallest unit of
matter
• Indivisible
“Plum Pudding” Model
JJ Thomson’s Model
Positively
charged
“pudding”
• Electrons are negative
• On the whole, atoms are
usually neutral
• Because of this, there must be
a positive charge within the
atom to balance the negative
electron
cathode ray tube demo
electrons have mass