2. We have studied the structure of an atom
previously and we know about the three main
subatomic particles: protons, neutrons and
electrons.
What makes the protons and electrons stay
together in an atom?
A force called the strong force opposes and
overcomes the force of repulsion between the
protons and holds the nucleus together. The
energy associated with the strong force is
called the binding energy. The electrons are
kept in orbit around the nucleus because there
is an electromagnetic field of attraction
between the positive charge of the protons and
the negative charge of the electrons.
3. In most cases, elements like to have an equal number
of protons and neutrons because this makes them
the most stable. Stable atoms have a binding energy
that is strong enough to hold the protons and
neutrons together.
However, an additional neutron or two
may upset the binding energy and cause
the atom to become unstable. In an
unstable atom, the nucleus changes by
giving off a neutron to get back to a
balanced state. As the unstable nucleus
changes, it gives off radiation and is said
to be radioactive.
4. All elements with atomic numbers greater than 83 are radioisotopes
meaning that these elements have unstable nuclei and are radioactive.
Elements with atomic numbers of 83 and less, have isotopes (stable
nucleus) and most have at least one radioisotope (unstable nucleus).
Examples of chemical elements with radioactive atoms include
uranium, plutonium & radium. As atoms give out particles or rays they
change into atoms of simpler elements. For example, uranium changes
to thorium. This change is called radioactive decay. When there is a
change in the nucleus and an element changes into another
element, it is called transmutation. It happens at different speeds or
rates for different radioactive elements.
changes to
Radioactivity can be dangerous since it harms living things. But under
controlled conditions it is very useful in medicine and scientific
research.
5. When alpha or beta particles (radiation) are emitted by a radioisotope, they
collide with surrounding atoms and make them move faster. In other
words, the temperature rises as nuclear energy is transformed into thermal
energy (heat).
The diagram shows what can happen
if a neutron strikes and penetrates a
nucleus of uranium-235. The nucleus
becomes highly unstable and splits
into two lighter nuclei. This splitting
process is called nuclear fission. If
emitted neutrons go on to split other
nuclei, the result is a chain reaction.
Whenever particles penetrate and
change a nucleus, this is called
nuclear reaction.
6. During the process of fission, bits of
matter cease to exist and become vast
quantities of energy instead. The energy
produced is given by ΔE = Δmc 2 where
Δm is the lost mass in kg and c is the
speed of light in m/s.
Albert Einstein postulated this theory
that mass could be converted into
energy.
7. A nuclear reactor is a device or a furnace
where controlled fission chain reaction is
carried out and the energy released is used
to generate electricity.
The reactor uses Uranium rods as fuel, and
the heat is generated by nuclear
fission: neutrons smash into the nucleus
of the uranium atoms, which split roughly
in half and release energy in the form of
heat. In nuclear reactions, the energy released per atom is around a million times
greater than that from a chemical change such as burning.
Carbon dioxide gas or water is pumped through the reactor to take the heat
away, this then heats water to make steam. The steam drives turbines which
drive generators. Modern nuclear power stations use the same type
of turbines and generators as conventional power stations.
8. The Sun and other stars are mostly
made of Hydrogen. At the centre of
the Sun, tremendous temperatures
and pressures make the hydrogen
nuclei collide at such high speeds
that they join together (fuse) to form
helium nuclei. As this happens, huge
amounts of energy are released as
heat and light. The energy travels up
to the Sun’s glowing surface and
then passes through space to reach
Earth.
