IGCSEPhysicsCambridge (CIE)Revision Notes5. Nuclear PhysicsThe Nuclear Model of the AtomProtons, Neutrons & Electrons

Protons, Neutrons & Electrons (Cambridge (CIE) IGCSE Physics)

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Written by: Ashika

Reviewed by: Caroline Carroll

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 

proton space equals space fraction numerator 1.6 cross times 10 to the power of negative 19 end exponent space straight C over denominator 1.6 cross times 10 to the power of negative 19 end exponent space straight C end fraction space equals space plus 1

  • The relative charge of an electron is

electron space equals space fraction numerator negative 1.6 cross times 10 to the power of negative 19 end exponent space straight C over denominator 1.6 cross times 10 to the power of negative 19 end exponent space straight C end fraction space equals space minus 1

  • The relative charge of a neutron is

neutron space equals space fraction numerator 0 space straight C over denominator 1.6 cross times 10 to the power of negative 19 end exponent space straight C end fraction space equals space 0

Table of relative charges

Particle

Relative charge

Proton

+1

Neutron

0

Electron

−1

Examiner Tips and Tricks

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

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: 

nuclear space charge space equals space number space of space protons space in space the space nucleus space cross times space relative space charge space of space straight a space 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 Cr presubscript 24 presuperscript 52 ? 

 

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

straight X presubscript straight Z presuperscript straight A

 

  • This chromium nucleus has 24 protons

Cr presubscript 24 presuperscript 52

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

Examiner Tips and Tricks

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

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: 

relative space mass space of space atom space equals space number space of space protons space and space neutrons space in space the space nucleus space cross times space relative space mass space of space straight a space proton space or space straight a space 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

Particle

Relative mass

Proton

1

Neutron

1

Electron

1 over 2000 negligible

Worked Example

What is the relative mass of the chromium nucleus  Cr presubscript 24 presuperscript 52?

 

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

straight X presubscript straight Z presuperscript straight A

 

  • This chromium nucleus has 52 protons and neutrons

Cr presubscript 24 presuperscript 52

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

Examiner Tips and Tricks

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.

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    Ashika

    Author: Ashika

    Expertise: Physics Project Lead

    Ashika graduated with a first-class Physics degree from Manchester University and, having worked as a software engineer, focused on Physics education, creating engaging content to help students across all levels. Now an experienced GCSE and A Level Physics and Maths tutor, Ashika helps to grow and improve our Physics resources.

    Caroline Carroll

    Author: Caroline Carroll

    Expertise: Physics Subject Lead

    Caroline graduated from the University of Nottingham with a degree in Chemistry and Molecular Physics. She spent several years working as an Industrial Chemist in the automotive industry before retraining to teach. Caroline has over 12 years of experience teaching GCSE and A-level chemistry and physics. She is passionate about creating high-quality resources to help students achieve their full potential.