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.
• In A2/A-level physics, 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.

• 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

                                                                      Mass

• 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.

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). This is about the size of a small nucleus. It acts upon particles made up of quarks and has the same effect on two protons, two neutrons or a proton and a neutron. It is an attractive force and binds hadrons (particles made up of quarks) together. However at separations 0.5 fm apart and smaller, it acts as a repulsive force that acts to prevent neutrons and protons being pushed into each other.

Radioactive Decay

• Radioactivity was first discovered in 1896 by Henri Becquerel. It is when the nucleus of an atom ‘decays’ to produce a more stable isotope. Radiation comes in 3 types: Alpha, Beta and Gamma.
• Alpha radiation is when a nucleus simply has too many nucleons. All atoms with proton number above 82 decay by alpha emission. An alpha particle is 2 protons and 2 neutrons (a helium nucleus).
• On the right is the equation for alpha emission. Notice that the proton number decreases by 2 and the mass number decreases by 4, because the proton number has changed, the element has changed (denoted by the change from x to y).

• Beta radiation is when a proton changes into a neutron or a neutron changes into a proton in the nucleus. If a neutron changes into a proton then a B- (beta minus particle; a fast moving high energy electron) is emitted. In this case, the proton number increases by 1 but the mass number is the same.

• In B+(beta plus; a fast moving high energy positron) decay a proton changes into a neutron. In this case the proton number is decreased by 1 but still the mass number stays the same.

• It is easy to work out whether a B- or B+ particle is being emitted by looking at the conservation of charge. Charge must be the same on the left hand side of the equation as on the right hand side. For example in B- decay on the left hand side the proton number is X, on the right hand side you have X+1 which means you have 1 extra so you must get rid of this by having a -1 charge on the electron. This way you know it must be a B- particle.

• Another particle is also emitted with beta decay. When scientists first started experimenting with decay they noticed that beta particles were emitted with different kinetic energies up to a maximum that depended on the isotope. This puzzled them because each nucleus would always lose a certain amount of energy during each decay which meant that the electrons should have a certain energy. They said that the extra energy must be carried away by another particle called the neutrino (or anti neutrino). In B+ decay a neutrino carries away the extra energy whereas in B- a neutrino carries away the extra energy (I remember this by thinking, if the B particle is matter then the neutrino is anti-matter and vice versa – it actually comes from pair production but there is more on this later on). Their hypothesis was proved 20 years later.

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

References:

1. https://getrevising.co.uk/resources/matter_and_radiation

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