Atomic Structure (OCR A Level Physics)

• The atoms of all elements are made up of three types of particles: protons, neutrons and electrons.

Protons and neutrons are found in the nucleus of an atom while electrons orbit the nucleus

• The properties of each particle in SI units are shown in the table below:

• It is important to know the relative masses and charges that protons, neutrons and electrons have in relation to each other

• The relative properties of each particle are shown in the table below:

• A stable atom is neutral (it has no charge)
• Since protons and electrons have the same charge, but opposite signs, a stable atom has an equal number of both for the overall charge to remain neutral

Relative Sizes

• From Rutherford's Alpha Scattering Experiment we know that most of an atom is made up of empty space

An atom: a small positive nucleus, surrounded by negative electrons

• The atom is around 100,000 times larger than the nucleus!
  • The diameter of an atom is 1 x 10⁻¹⁰ m
  • The diameter of a nucleus is 1 x 10⁻¹⁵ m
  • So, 1 x 10^-15 x 100,000 = 1 x 10⁻¹⁰

Closest Approach Method

• In the Rutherford scattering experiment, alpha particles are fired at a thin gold foil
• Some of the alpha particles are found to come straight back from the gold foil
• This indicates that there is electrostatic repulsion between the alpha particles and the gold nucleus

• At the point of closest approach, r, the repulsive force reduces the speed of the alpha particles to zero momentarily
• At this point, the initial kinetic energy of an alpha particle, E_k, is equal to electric potential energy, E_p
• The radius of the closest approach can be found be equating the initial kinetic energy to the electric potential energy

• Equating the two equations gives: