3.8 Nuclear Physics
3.8.1 Radioactivity
3.8.1.1 Rutherford Scattering
At first, atoms were thought to be the tiniest thing in the universe that could not be
divided. However, JJ Thomson discovered the electron whilst he was doing
experiments with cathode ray tubes. Cathode ray tubes are when a high voltage
power supply is used to send an electrical current through a vacuum, and a glow
can be seen in the tube because of something coming from the negative
electrode, which is called the cathode.
He studied the ‘cathode rays’ - which were emitted from the negative electrode -
and found that they were made of particles that were negatively charged and
had an extremely tiny (negligible) mass. JJ Thomas’ discovery of the electron led
to the ‘plum pudding’ model of the atom, which envisaged the atom as a ball of
positive charge with negative electrons embedded in it (ie the electrons are the
plums embedded throughout a spherical plum pudding).
Then some experiments by Rutherford and Marsden led to the plum pudding
model being discarded. They fired alpha particles (a kind of positively charged
radiation) at gold foil. The foil was extremely thin - only a few atoms across - and
most of the alpha particles passed straight through it. Some, however, were
scattered - some even bounced off the foil.
, Rutherford and Marsden then concluded that most of the atom is made up of
empty space (which is why most of the alpha particles passed straight through
the foil) with a small area of positive charge (which is why some of the alpha
particles were repelled and ‘bounced’ off the foil). Thus, the plum plum pudding
model was discarded and replaced by the nuclear model of the atom, whereby a
tiny nucleus contains the positive charge of the atom.
In the next major development Niels Bohr, a physicist, adapted the nuclear model
by showing that electrons are found moving around the nucleus at specific
distances. The theoretical calculations of Bohr agreed with experimental
observations. Later experiments led to the idea that the positive charge of any
nucleus could be subdivided into a whole number of smaller particles, each
particle having the same amount of positive charge. The name proton was given
to these particles.
In 1932, Chadwick proved the existence of neutrons within the nucleus. It had
already been observed that an atom’s mass was usually about twice its atomic
number. The discovery of protons and electrons could not explain this. Chadwick
was working with a radioactive substance, polonium. Radiation emitted from
polonium behaved in an unexpected way. Polonium seemed to be emitting
subatomic particles, each with mass, but they did not behave like protons.
Chadwick’s experiments revealed that this radiation consisted of uncharged
particles, each with the same mass as a proton. They must have come from the
polonium atoms. Thus, Chadwick’s experiments had shown the existence of
neutrons. This accounts for observations that the atomic mass of an atom is
higher than the mass of its protons.
3.8.1.2 ɑ, β and ɣ Radiation
ɑ Radiation
● Helium nucleus (two protons, two neutrons) → +2 charge
● When released, the atom loses two protons and two neutrons:
○ Atomic number of daughter nucleus is 2 less than that of parent
nucleus
3.8.1 Radioactivity
3.8.1.1 Rutherford Scattering
At first, atoms were thought to be the tiniest thing in the universe that could not be
divided. However, JJ Thomson discovered the electron whilst he was doing
experiments with cathode ray tubes. Cathode ray tubes are when a high voltage
power supply is used to send an electrical current through a vacuum, and a glow
can be seen in the tube because of something coming from the negative
electrode, which is called the cathode.
He studied the ‘cathode rays’ - which were emitted from the negative electrode -
and found that they were made of particles that were negatively charged and
had an extremely tiny (negligible) mass. JJ Thomas’ discovery of the electron led
to the ‘plum pudding’ model of the atom, which envisaged the atom as a ball of
positive charge with negative electrons embedded in it (ie the electrons are the
plums embedded throughout a spherical plum pudding).
Then some experiments by Rutherford and Marsden led to the plum pudding
model being discarded. They fired alpha particles (a kind of positively charged
radiation) at gold foil. The foil was extremely thin - only a few atoms across - and
most of the alpha particles passed straight through it. Some, however, were
scattered - some even bounced off the foil.
, Rutherford and Marsden then concluded that most of the atom is made up of
empty space (which is why most of the alpha particles passed straight through
the foil) with a small area of positive charge (which is why some of the alpha
particles were repelled and ‘bounced’ off the foil). Thus, the plum plum pudding
model was discarded and replaced by the nuclear model of the atom, whereby a
tiny nucleus contains the positive charge of the atom.
In the next major development Niels Bohr, a physicist, adapted the nuclear model
by showing that electrons are found moving around the nucleus at specific
distances. The theoretical calculations of Bohr agreed with experimental
observations. Later experiments led to the idea that the positive charge of any
nucleus could be subdivided into a whole number of smaller particles, each
particle having the same amount of positive charge. The name proton was given
to these particles.
In 1932, Chadwick proved the existence of neutrons within the nucleus. It had
already been observed that an atom’s mass was usually about twice its atomic
number. The discovery of protons and electrons could not explain this. Chadwick
was working with a radioactive substance, polonium. Radiation emitted from
polonium behaved in an unexpected way. Polonium seemed to be emitting
subatomic particles, each with mass, but they did not behave like protons.
Chadwick’s experiments revealed that this radiation consisted of uncharged
particles, each with the same mass as a proton. They must have come from the
polonium atoms. Thus, Chadwick’s experiments had shown the existence of
neutrons. This accounts for observations that the atomic mass of an atom is
higher than the mass of its protons.
3.8.1.2 ɑ, β and ɣ Radiation
ɑ Radiation
● Helium nucleus (two protons, two neutrons) → +2 charge
● When released, the atom loses two protons and two neutrons:
○ Atomic number of daughter nucleus is 2 less than that of parent
nucleus
