Protons, Neutrons & Electrons (Cambridge (CIE) IGCSE Physics): Revision Note

Relative charge

• Charge can be positive or negative

• Protons, neutrons and electrons have different properties and different charges

  • Protons have a positive charge

  • Electrons have a negative charge

  • Neutrons have no charge

• Relative charge can be used to compare these properties

  • Relative charge is the ratio of the charge of a particle compared to the fundamental charge 

  • Because relative charge is a ratio, it has no units

• Fundamental charge is equal to the magnitude of the charge on a proton and an electron = 1.6 × 10⁻¹⁹ C

• The relative charge of a proton is 

$\mathrm{proton}=\frac{1.6\times {10}^{-19}\mathrm{C}}{1.6\times {10}^{-19}\mathrm{C}}=+1$

• The relative charge of an electron is

$\mathrm{electron}=\frac{-1.6\times {10}^{-19}\mathrm{C}}{1.6\times {10}^{-19}\mathrm{C}}=-1$

• The relative charge of a neutron is

$\mathrm{neutron}=\frac{0\mathrm{C}}{1.6\times {10}^{-19}\mathrm{C}}=0$

Table of relative charges

Nuclear charge

Extended tier only

• Nuclear charge is the relative charge of the nucleus

• This is determined by the proton number (opens in a new tab) of the atom

• Relative charge is calculated by: 

$\mathrm{nuclear}\mathrm{charge}=\mathrm{number}\mathrm{of}\mathrm{protons}\mathrm{in}\mathrm{the}\mathrm{nucleus}\times \mathrm{relative}\mathrm{charge}\mathrm{of}\mathrm{a}\mathrm{proton}$

• An element of carbon has a proton number of 6

  • So the relative charge of carbon is 6 × (+1) = 6

Nuclear mass

Extended tier only

• Nuclear mass or the relative mass of an atom, is the relative mass of the nucleus

  • This is because the mass of electrons orbiting the nucleus is negligible

• A relative mass of 1 = 1.67 × 10^-27 kg

• The relative mass of an atom is determined by the nucleon number of the atom

• It is calculated by: 

$\mathrm{relative}\mathrm{mass}\mathrm{of}\mathrm{atom}=\mathrm{number}\mathrm{of}\mathrm{protons}\mathrm{and}\mathrm{neutrons}\mathrm{in}\mathrm{the}\mathrm{nucleus}\times \mathrm{relative}\mathrm{mass}\mathrm{of}\mathrm{a}\mathrm{proton}\mathrm{or}\mathrm{a}\mathrm{neutron}$

• An element of carbon has a nucleon number of 12

  • So the relative mass of carbon is 12 × 1 = 12 atomic mass units or amu

Table of relative mass