Atomic Structure (OCR A Level Physics): Revision Note

Atomic Structure

• 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:

• An atom is neutral (it has no charge)

• Since protons and electrons have the same charge, but opposite signs, an atom has an equal number of both for the overall charge to remain neutral

Relative Sizes of the Atom and Nucleus

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 × 10⁻¹⁰ m

  • The diameter of a nucleus is 1 × 10⁻¹⁵ m

  • So, 1 × 10^-15 × 100,000 = 1 × 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

$E_k=\frac{1}{2}m{v}^2$

• 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

$E_p=\frac{Qq}{4\pi {\epsilon }_{0}r}$

• Equating the two equations gives:

$E_k=E_p=\frac{1}{2}m{v}^2=\frac{Qq}{4\pi {\epsilon }_{0}r}$