CIE AS Physics (9702) 2019-2021

Revision Notes

13.1.2 Nucleon & Proton Number

Nucleon & Proton Number

  • The atomic symbol of an element is used to describe the constituents of the nuclei
  • An example of this notation for Lithium is:

Mass and atomic number, downloadable AS & A Level Physics revision notes

Atomic symbol for Lithium

  • When given an atomic symbol, you can figure out the number of protons, neutrons and electrons in the atom:
    • Protons: The atomic number
    • Electrons: Atoms are neutrals, so the number of negative electrons is equal to the number of positive protons. Therefore, this is also the atomic number
    • Neutrons: Subtract the proton number from the mass number
  • For the lithium atom, these numbers would be:
    • Protons: 3
    • Electrons: 3
    • Neutrons: 7 − 3 = 4
  • The term nucleon is the used to mean a particle in the nucleus – i.e. a proton or neutron
  • The term nuclide is used to refer to a nucleus with a specific combination of protons and neutrons

Worked example

Worked example - nucleon and proton number, downloadable AS & A Level Physics revision notes

 

Isotopes

  • Although all atoms of the same element always have the same number of protons (and hence electrons), the number of neutrons can vary
  • An isotope is an atom (of the same element) that has an equal number of protons but different number of neutrons
  • The isotopes of hydrogen are deuterium and tritium:

Isotopes of Hydrogen, downloadable AS & A Level Physics revision notes

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

  • Remember, the neutron number of an atom is found by subtracting the proton number from the nucleon number
  • Since nucleon number includes the number of neutrons, an isotope of an element will also have a different nucleon/mass number
  • Since isotopes have an imbalance of neutrons and protons, they are unstable. This means they constantly decay and emit radiation to achieve a more stable form
  • This can happen from anywhere between a few nanoseconds to 100,000 years

Worked example

Worked example - isotopes, downloadable AS & A Level Physics revision notes

  • Isotopes are nuclei with the same number of protons but different number of neutrons
    • The nucleon number is the sum of the protons and neutrons
    • Therefore, an isotope has a different nucleon number too
  • The first nucleus:
    • Nucleon number: 37
    • Neutrons: 20
    • Protons = 37 − 20 = 17
  • The second nucleus:
    • Nucleon number: 35
    • Neutrons: 18
    • Protons = 35 − 18 = 17
  • Therefore, they have the same number of protons but different numbers of neutrons and are isotopes of each other
  • This refers to row B

 

AZX Notation

  • Atomic symbols are written in a specific notation called AZX notation

AZX Notation, downloadable AS & A Level Physics revision notes

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

  • The top number A represents the nucleon number or the mass number
    • Nucleon number (A) = total number of protons and neutrons in the nucleus
  • The lower number Z represents the proton or atomic number
    • Proton number (Z) = total number of protons in the nucleus

 

  • Note: In Chemistry the nucleon number is referred to as the mass number and the proton number as the atomic number. The periodic table is ordered by atomic number

Conservation of Nucleon Number & Charge

  • Nuclear processes such as fission and fusion are represented using nuclear equations (similar to chemical reactions in chemistry)
  • The number of protons and neutrons in atom is known as its constituents
  • For example:

Nuclear reaction equation, downloadable AS & A Level Physics revision notes

Nuclear fission equation

  • The above equation represents a fission reaction in which a Uranium nucleus is hit with a neutron and splits into two smaller nuclei – a Strontium nucleus and Xenon nucleus, releasing two neutrons in the process
  • In nuclear equations, the nucleon number and charge are always conserved
  • This means that the sum of the nucleons and charge on the left hand side must equal the sum of the number of nucleons and charge on the right hand side
  • In the above equation, the sum of the nucleon (top) numbers on both sides are equal

235 + 1 = 236 = 90 + 144 + 2 × 1

  • The same is true for the proton (bottom) numbers

92 + 0 = 92 = 38 + 54 + 2 × 0

  • By balancing equations in this way, you can determine the nucleon, proton number or the number of missing elements
  • For example:

Nuclear equation example, downloadable AS & A Level Physics revision notes

Balancing the number of nucleons shows that 3 neutrons must be released in the reaction

 

Worked example

Worked example - Nuclear equations, downloadable AS & A Level Physics revision notes

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