Atomic Structure (AQA A Level Physics): Revision Note

Atomic structure

• All matter is made from atoms

• Atoms are made up of three types of particles:

  • Protons

  • Neutrons

  • Electrons

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

• 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

Specific charge

• In physics, ‘specific’ refers to the characteristic of a property of a substance

  • It is usually stated as a ratio i.e. per unit mass, length, area, volume etc.

  • For example, specific heat capacity refers to the thermal energy per unit mass of a substance

• In atomic physics, specific charge is defined as:

The ratio of the total charge of a particle to its mass

• It can be calculated using the equation:

$\mathrm{specific}\mathrm{charge}=\frac{\mathrm{charge}}{\mathrm{mass}}=\frac{Q}{m}$

• Specific charge is measured in units of coulombs per kilogram (C kg^–1)

• Values for the specific charge of the electron and proton are given on the datasheet as the ‘charge / mass ratio’

  • The specific charge of the electron $=\frac{e}{m_e}=1.76\times {10}^{11}\mathrm{C}{\mathrm{kg}}^{-1}$

  • The specific charge of the proton $=\frac{e}{m_p}=9.58\times {10}^7\mathrm{C}{\mathrm{kg}}^{-1}$

Calculating specific charge

• The specific charge of an ion or a nucleus can also be found

  • Charge of a proton or electron, e = 1.60 × 10^–19 C

  • Mass of a proton, m_p (or neutron, m_n) = 1.67 × 10^–27 kg

• To calculate the specific charge of an ion:

  • Charge = Total number of electrons added / removed × (1.60 × 10^–19 C)

  • Mass = Total number of nucleons × (1.67 × 10^–27 kg)

• To calculate the specific charge of a nucleus:

  • Charge = Total charge of the protons × (1.60 × 10^–19 C)

  • Mass = Total number of nucleons × (1.67 × 10^–27 kg)

• The number of nucleons is given by the mass number of the ion or nucleus