Protons, Neutrons & Electrons (Cambridge International IGCSE Physics)

Relative charge

Spec point — 5.1.3 · State the relative charges of protons, neutrons and electrons as +1, 0 and –1 respectively

• 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 × 10⁻¹⁹C

• The relative charge of a proton is

$\mathrm{proton}=\frac{1.6\times10^{-19}C}{1.6\times10^{-19}C}=+1$

• The relative charge of an electron is

$\mathrm{electron}=\frac{-1.6\times10^{-19}C}{1.6\times10^{-19}C}=-1$

• The relative charge of a neutron is

$\mathrm{neutron}=\frac{0C}{1.6\times10^{-19}C}=0$

Table of relative charges

Exam Hint You don't need to know how to calculate relative charge, but you do need to understand what it is. You do also need to remember what the relative charges are for each subatomic particle.

Nuclear charge

Spec point — 5.1.3 · Know the relationship between the proton number and the relative charge on a nucleus

Extended Tier Only

• Nuclear charge is the relative charge of the nucleus
• This is determined by the proton number of the atom
• Relative charge is calculated by:

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

• An element of carbon has a proton number of 6

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

Worked Example What is the relative charge of the chromium nucleus ${}_{24}^{52}\mathrm{Cr} ?$

[1]

Answer:

Step 1: Determine the number of protons

• The number of protons is the proton number
• This is the Z number in the AZX notation

$\mathrm{X} pre_{Z} pre^{A}$

• This chromium nucleus has 24 protons

${}_{24}^{52}\mathrm{Cr}$

Step 2: State the relative mass of 1 proton

• 1 proton has a relative charge of +1

Step 3: Multiply relative charge of 1 proton by the number of protons

• This nucleus of chromium therefore has a relative charge of +24 [1 mark]

Exam Hint Charge can be either positive (+) or negative (–). Usually, when a number is positive, we don't write the + sign, but when dealing with relative charge, we do.

Nuclear mass

Spec point — 5.1.3 · Know the relationship between the nucleon number and the relative mass of a nucleus

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:

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

• 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

Worked Example What is the relative mass of the chromium nucleus ${}_{24}^{52}\mathrm{Cr}$?

[1]

Answer:

Step 1: Determine the number of protons and neutrons

• The number of protons and neutrons is the mass (nucleon) number
• This is the A number in the AZX notation

$\mathrm{X} pre_{Z} pre^{A}$

• This chromium nucleus has 52 protons and neutrons

${}_{24}^{52}\mathrm{Cr}$

Step 2: State the relative mass of 1 proton and 1 neutron

• 1 proton has a relative mass of 1
• 1 neutron has a relative mass of 1

Step 3: Multiply relative charge of 1 proton and 1 neutron by number of protons and neutrons

• This nucleus of chromium therefore has a relative mass of 52 [1 mark]

Exam Hint The relative mass of a nucleus only includes the protons and neutrons. However, this is pretty much the relative mass of the whole atom because electrons have negligible (very little) mass in comparison to the proton and neutron.