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Nucleon & Proton Number (CIE AS Physics)

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Nucleon & proton number

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

Li presubscript 3 presuperscript 7

  • Where:
    • Li = lithium
    • 7 = mass number or nucleon number
    • 3 = atomic number or proton number
  • The mass number is the number of particles in the nucleus
    • The nucleus contains protons plus neutrons
  • The atomic number is the number of protons in the atom
    • Since atoms are neutral, this is also equal to the number of electrons in the atom

  • When given an atomic symbol, you can therefore 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 nucleon 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

The atom Ir presubscript 77 presuperscript 192 is a neutral atom.

State the number of protons, neutrons and electrons in this atom.

  Protons Neutrons Electrons
A 77 192 77
B 155 77 77
C 192 77 192
D 77 115 77

Answer: D

Step 1: Determine the nucleon and atomic numbers

  • Nucleon number = 192
  • Atomic number = 77

Step 2: Determine the number of protons

  • Number of protons = atomic number
  • Number of protons = 77

Step 3: Determine the number of neutrons

  • Number of neutrons = nucleon number − atomic number
  • Number of neutrons = 192 − 77
  • Number of neutrons = 115

Step 3: Determine the number of electrons

  • Number of electrons = number of protons
  • Number of electrons = 77

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

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 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

One of the rows in the table shows a pair of nuclei that are isotopes of one another.

Which row shows the correct isotopes?

  Nucleon number Number of neutrons
A

39

35

19

22

B

37

35

20

18

C

37

35

18

20

D

35

35

20

18

Answer: B

Step 1: Recall the properties of isotopes

  • Isotopes are nuclei with the same number of protons but a different number of neutrons
  • The nucleon number (mass number) is the total number of particles in the nucleus (protons + neutrons)
  • Therefore, isotopes have a different nucleon number too

Step 2: Calculate the number of protons in the first nucleus

  • Nucleon number = 37
  • Neutrons = 20
  • Protons = nucleon number − neutrons
  • Protons = 37 − 20 = 17

Step 3: Calculate the number of protons in the second nucleus

  • Nucleon number = 35
  • Neutrons = 18
  • Protons = 35 − 18 = 17

Step 4: Conclusion

  • The nuclei in answer B have the same number of protons and different numbers of neutrons, therefore these nuclei are isotopes of one another

AZX notation

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

straight X presubscript straight Z presuperscript straight A

  • Where:
    • X = chemical symbol of the element
    • A = nucleon (mass) number
    • Z = proton (atomic) number

  • The 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

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Leander

Author: Leander

Expertise: Physics

Leander graduated with First-class honours in Science and Education from Sheffield Hallam University. She won the prestigious Lord Robert Winston Solomon Lipson Prize in recognition of her dedication to science and teaching excellence. After teaching and tutoring both science and maths students, Leander now brings this passion for helping young people reach their potential to her work at SME.