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Atomic structure

The Rutherford-Bohr model of an atom

The Rutherford-Bohr model of an atom
The Rutherford-Bohr model of an atom

Atoms consist of:

  • Nucleus: contains positive protons (p) and neutral neutrons (n)
  • Electrons: circle the nucleus within energy “shells”

Describing an atom

AZX notation

Atoms are displayed in the format shown to the left where:

A = mass number (p + n)
Z = atomic number (protons)
X = chemical symbol of the atom

The neutrons and protons (collectively called nucleons) give the atom its mass. This isn’t the actual mass but that relative to other atoms.

1 atomic mass unit (amu) = 1/12 the mass of a carbon-12 atom

The amu’s of different components of the atoms are shown in the table below:

 Relative massChargeSymbol

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Electron shells

The number of electron shells orbiting the nucleus is different depending upon the number of electrons in the atom. A very simplistic model is that each shell has a letter symbol and a maximum number of electrons it can hold calculated by 2n2 where n = shell number.

Electron shells
The maximum number of electrons a shell can hold is 2n2
Shell numberLetter symbolMaximum number of electrons
1K2 x 12 = 2
2L2 x 22 = 8
3M2 x 32 = 18


Types of electrons

Electrons are either bound or free.

Bound electrons: These are the electrons that are held in orbit around the nucleus in the electron shells by the attractive force of the positive nucleus. The binding energy is the positive energy required to overcome the pull of the nucleus and release the electron from the shell. This is of the same magnitude as the actual (negative) energy of the electron that is released if the electron is freed.

Free electrons: These are the electrons that are not bound in an electron shell around a nucleus. They have a kinetic energy of:

Kinetic energy = ½mv2

m = mass
v = velocity

The actual binding energy of electrons is expressed in electron volts (eV) or keV (1keV = 1000 eV)

1 eV = 1.6022 x 10-19 joules

  Key points

  • Increase in the atomic number = increase in the binding energy of the electrons (there are more protons and, therefore, more energy is needed to release the electrons from the greater positive pull).
  • Increase in the distance between the nucleus and the electron = decrease in the binding energy of the electron (decrease in the positive pull of the protons in the nucleus).

Nuclear stability

The nucleus is composed of protons and neutrons. The protons repel each other (electrostatic force) but the nucleus is kept held together by the strong nuclear force.

Strong nuclear force (aka strong interaction): There is a strong force of attraction at distances between nucleons of 1 x 10-15 m (i.e. 1 femtometre, fm) which changes to a repulsive force at <0.7 x 10-15 m. The nucleons are kept apart at a distance of 1 to 2 x 10-15 m, the distance at which there is the greatest attraction.

Electrostatic force (aka coulomb force): this is the force of repulsion between protons. At distances of 10-15 to 10-16 m the strong attractive interaction (strong nuclear force) is much greater than the repulsive electrostatic force and the nucleus is held together.


Segrè chart

The Segrè chart
The Segrè chart

As the atomic number increases (i.e. the number of protons) more neutrons are required to prevent the electrostatic forces pushing the protons apart and to keep the nucleus stable. The Segré chart shows the proportion of neutrons needed to keep the nucleus stable as the number of protons increases (the “line of stability”).

If an atom has too many or too few neutrons and does not lie upon the “line of stability”, it becomes unstable and decays to a more stable form. This is the basis of radioactivity and is discussed next in the “electromagnetic radiation” chapter.

Σ  Summary

  • An atom is composed of neutrons, protons and electrons
  • Neutrons and protons form the nucleus and, collectively, are called nucleons
  • A neutron has a mass of 1 and a charge of 0
  • A proton has a mass of 1 and a charge of +1
  • An electron has a mass of 0.0005 and a charge of -1
  • The mass number (A) of an atom is the number of protons and neutrons
  • The atomic number (Z) is the number of protons
  • Electrons are held in electron shells that each hold a maximum number of electrons
    • Max no. electrons per shell = 2n2, where n = shell number
  • Electrons have a binding energy that is the same as their actual negative energy.
    • Binding energy = the positive energy required to release the electron from its shell = the negative energy released by electron when it is freed
  • The farther away from the nucleus the electron is the smaller its binding energy
  • The higher the atomic number, the greater the binding energy