Matter and Radiation

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  • Created on: 17-11-13 13:12
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Matter and Radiation
The Structure of The Atom
Atoms are tiny; the diameter of an atom is around 10e-11m (this notation means 10 to the
power of -11 meters). Ernst Rutherford set up an experiment in which he fired alpha particles
at gold foil and measured the deflection of the alpha particles. This experiment showed that
the atom has a positive nucleus which consists of protons and neutrons and negative electrons
that surround the nucleus in shells.
The word `nucleon' means proton or neutron in the nucleus. When we talk about the protons
and neutrons in a nucleus we collectively call them nucleons.
Most of the mass in an atom is centred in the nucleus. This is because protons and neutrons
have a much bigger mass than the electrons. The nucleus is around 10e-15m across, which is
only about 0.00001 times the diameter of the atom itself.
Charge (C) Charge compared to Mass (kg) Mass compared to
that of a proton that of a proton
Proton 1.6x10e-19 1 1.67x10e-27 1
Neutron 0 0 1.67x10e-27 1.0004
Electron -1.6x10e-19 -1 9.11x10-31 1/1836
You can see from the table that although the masses are very similar, the neutron has slightly more mass than the
proton, whereas the electron has a much smaller mass than either of the nucleons. The electron has equal but
opposite charge to a proton. Atoms have equal numbers of protons to electrons, this is what gives the atom its
overall neutral charge (as the +ve and ­ve cancel out).
Atoms of the same element have the same amount of protons (this is what defines the element; if you look at the
periodic table each new element has 1 more proton than the previous). The proton number is usually called the
atomic number and is denoted the symbol Z. Isotopes are atoms with the same number of protons (same element)
but different number of neutrons. This means that isotopes have different mass numbers (labelled A) but same
atomic numbers.
Number of neutrons = mass number ­ atomic number (A-Z)
Specific Charge
Electrons have a very small mass compared to the proton mass but they have equal magnitude of charge. The
charge/mass ratio is called the specific charge and shows much charge there is to unit mass. The electron is tiny but
has a big charge; this means that it has a lot of charge for every unit (kg) of mass. The proton has a greater mass
however so the amount of charge there is to mass is smaller.
Specific Charge = Charge (C)
Mass (kg)
The electron has the greatest specific charge of any particle.
Atoms overall have no charge (as the protons cancel out the electrons). This means they have 0 specific charge. Ions
on the other hand have an overall charge because they've lost/gained electrons.
Specific charge of ion = Charge due to extra/reduced electrons
You can also work out the specific charge of the nucleus of an atom = No of protons x proton charge
No of nucleons x nucleon mass
Knowing the specific charge of a particle is useful because it can help you predict the behaviour of the said particle in
an electric or magnetic field. Particles with the same specific charge will be deflected by the same amount.

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The Strong Nuclear Force
There are 4 forces that act on matter. In order of decreasing strength they are: Strong nuclear force,
electromagnetic force, weak nuclear force and gravity. The strong force is responsible for holding nuclei together so
that we don't disintegrate. It must be stronger to overcome the electromagnetic force otherwise the positive
repulsion from the protons would blow the nucleus apart.
The strong nuclear force has a small range no more than 3-4
femtometers (fm ­ one fm is 10e-15 m).…read more

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Gamma radiation is an electromagnetic wave emitted by a nucleus with too much energy after an alpha or beta
emission.…read more


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