Late 1800s/ Early 1900s
• Experiments were conducted using electricity and
matter
• Use a cathode-ray Tube
At each end of the cathode -ray tube
electrodes were connected to electricity.
• Particles pass through the cathode, the negative
side of the tube, and travel to the anode, the
positive end of the tube.
• JJ Thomson - 1897
Found out
• When current was passed through the tube
the opposite end of the tube glowed.
• Hypothesized that the glow was caused by
particles – a cathode ray.
Put a paddle wheel in the way of
the cathode ray
• Found that the paddle wheel moved away
from the cathode towards the anode.
• Hypothesized that the particles had mass.
Magnetic field experiments
• The ray was deflected away from a
negatively charged object.
• Opposites attract
• The particle had a negative charge.
The electron was discovered
Credit to: JJ Thomson
• Cathode ray experiments showed that the
atom was divisible.
• Particles that composed the cathode- ray
were negative. Called electrons.
• Electrons are present in all elements
– Knew this because he used various elements to
do the experiment and got the same results.
Millikan
• Large negative charge and a small mass.
– Millikan did experiments in 1909 to show the
mass of the electron to be
9.109 x 10 –31kg
• Or 1/1837th of a hydrogen atom
• Obtained the first accurate measurement of an
electron’s charge.
• “Oil Drop” experiment
Inferences were made
• Because atoms are neutral, there must be a
positive subatomic particle to balance out
the negatively charged electron
• Because electrons have less mass than
atoms, atoms must contain other subatomic
particles.
Nucleus
• Rutherford, Greiger, Marsden in 1911
• Famous “Gold Foil” experiment
• Bombarded gold foil with large alpha particles,
– Expected the particles to go straight through the foil
– Some did, some deflected
– A great dense mass had to be there to deflect the large
alpha particle
Rutherford Concluded
• There must have been a very small but powerful
force in the atom.
• Force must be densely packed matter with a
positive charge.
• Nucleus very small in size compared to the entire
atom.
• Summarized- most of the mass is in the nucleus
and the nucleus is a very small part of the atom.
The atom was made up of mostly empty space.
Nucleus
• Protons and neutrons in nucleus
• Electrons around nucleus
• Atoms are electrically neutral therefore, the
number of protons must balance out the
number of electrons.
• # Protons = #Electrons in a neutral atom
Proton importance
• The # of protons determines the atoms
identity.
• Each element has a specific number of
protons. The other two can vary.
• M:\downloaded clips\clips chapter 3\3.2
history size exceptions.asx
Atomic mass
• All of the mass of an atom is in its nucleus
• The atomic mass minus the number of
protons will give you the number of
neutrons.
• Atomic mass – atomic number = number of
neutrons
Nuclear forces
• Generally particles that have the same charge repel each
other.
• Not true of protons
• When two or more protons are close they are attracted to
each other.
• The same situation exists for neutrons
• These short range proton-neutron, proton- proton, neutronneutron forces hold the nuclear particles together are called
nuclear forces.
• M:\downloaded clips\clips chapter 3\Nuclear_Stability
nuclear forces 3.2.asx
Atomic size
• Electrons are in a “cloud” around the nucleus.
• Radius of the atom is from the center of the
nucleus to the outside of the cloud.
• Measured in Pico meters pm
• 1pm = 1x10-12 m
• Atom would be about 40-270 pm
• Nucleus would be about 0.001 pm
• Density of nucleus is about 2 x10 8 tons /cm3 !!!!!
Vocabulary
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Atom
Nucleus
Electron
Proton
neutron