CIE IGCSE Physics

Revision Notes

5.1.2 Nucleus

Protons & Neutrons

  • Atoms are made up of three different particles:

 

atom-proton-neutron-electron, IGCSE & GCSE Physics revision notes

Protons and neutrons are found in the nucleus of an atom

 

The properties of each of these particles is shown in the table below:

Nucleus table, IGCSE & GCSE Physics revision notes

  • The atom shown in the above diagram can also be represented using an atomic symbol:

 

Atomic symbol, IGCSE & GCSE Physics revision notes

Atomic symbols, like the one above, describe the constituents of nuclei

 

  • The top number is called the nucleon number, A, and is equal to the total number of particles (protons and neutrons) in the nucleus
  • The lower number is called the proton number, Z, and is equal to the total number of protons in the nucleus

(Note: Chemists refer to the nucleon number as the mass number, and the proton number as the atomic number)

  • When given an atomic symbol, you can figure out the total number of protons, neutrons and electrons in the atom:
    • Protons: The number of protons is equal to the proton number
    • Electrons: Atoms are neutral, and so in a neutral atom the number of negative electrons must be equal to the number of positive protons
    • Neutrons: The number of neutrons can be found by subtracting the proton number from the nucleon number
  • The term nucleon is used to mean a particle in the nucleus – ie. either a proton or a neutron
  • The term nuclide is used to refer to a nucleus with a specific combination of protons and neutrons

 

Isotopes

  • Although all atoms of the same element always have the same number of protons (and hence electrons), the number of neutrons can vary:

 

Three isotopes of hydrogen, IGCSE & GCSE Physics revision notes

The three atoms shown above are all forms of hydrogen, but they each have different numbers of neutrons

 

  • The number of neutrons in an atom does not affect the chemical properties of an atom, only its mass. Such atoms are called isotopes:
    Isotopes are atoms (of the same element) that have equal numbers of protons but different numbers of neutrons

 

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Jenna studied at Cardiff University before training to become a science teacher at the University of Bath specialising in Biology (although she loves teaching all three sciences at GCSE level!). Teaching is her passion, and with 10 years experience teaching across a wide range of specifications – from GCSE and A Level Biology in the UK to IGCSE and IB Biology internationally – she knows what is required to pass those Biology exams.
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