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Studying Atoms
1899-Modern Atomic Theory
Book pages 104-105 & 113-116
ISN pg 43
Rutherford’s Atomic Theory
• 1899, Rutherford
discovered that
uranium emits fastmoving particles
that have a positive
charge called alpha
particles
Rutherford’s Atomic Theory
• Hypothesis: (asked student- Marsden, to find
out what happens to alpha particles when
they pass through a thin sheet of gold)
Predicted that most particles would travel in a
straight path from their source to a screen
that lit up when struck.
Rutherford’s Atomic Theory
• The Gold Foil Experiment
– Aimed narrow beam of alpha particles at gold, the
material around the gold produced a flash of light
when struck by fast-moving alpha particles
– Could figure out path of alpha particles by
observing the flash after they had passed through
the gold
– Did NOT support Rutherford’s prediction
Rutherford’s Atomic Theory
• Gold Foil Experiment continued…
– More particles were deflected than he expected
– About 1 out of every 20,000 was deflected by
more than 90 degrees
– Some alpha particles behaved as though they had
struck an object and bounced straight back
Gold Foil Experiment
Rutherford’s Atomic Theory
• Discovery of the Nucleus
– Alpha particles whose paths were deflected must
have come close to another charged object
– The closer they came, the greater the deflection
– Concluded: the positive charge of an atom is not
evenly spread throughout the atom, it is
concentrated in a very small central area- nucleus
– Nucleus: a dense, positively charged mass located
in the center of the atom
Rutherford’s Atomic Theory
• According to Rutherford’s model, all of an
atom’s positive charge is concentrated in its
nucleus
Properties of Subatomic Particles
• Neutrons
– 1932, English physicist J. Chadwick designed an
experiment to show that neutrons exist;
concluded particles were neutral because a
charged object did not deflect their path
– A neutral subatomic particle that is found in the
nucleus of an atom
– Mass is almost exactly equal to that of a proton
Bohr’s Model of the Atom
• Bohr was a Danish physicist who worked with
Rutherford
• Bohr agreed with Rutherford’s model, but
Bohr’s model focused on the electrons
• Energy Levels
– In Bohr’s model, electrons move with constant
speed in fixed orbits around the nucleus (like
planets around a sun)
Bohr’s Model of the Atom
• Energy Levels
– Each electron in an atom has a specific amount of
energy
– If an atom gains or loses energy, the energy of an
electron can change
– The possible energies that electrons in an atom
can have are called energy levels
– An electron cannot exist between energy levels
Bohr’s Model of the Atom
• Energy Levels
– An electron in an atom can move from one energy
level to another when the atom gains or loses
energy
– The size of the jump between energy levels
determines the amount of energy gained or lost
• Evidence for Energy Levels
– Scientists can measure the energy gained or
released when an electron absorbs or releases
energy and moves to higher or lower energy levels
Bohr’s Model of the Atom
Electron Cloud Model
• An electron cloud is a visual model of the
most likely locations for electrons in an atom
• The cloud is denser at those locations where
the probability of finding an electron is high
• Scientists use the electron
cloud model to describe the
possible locations of electrons
around the nucleus