Nucleon & Proton Number (Cambridge International AS Physics)

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:

${}_{3}^{7}\mathrm{Li}$

• 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 ${}_{77}^{192}\mathrm{Ir}$ 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 4: Determine the number of electrons

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

Isotopes

Spec point — 11.1.4

• 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

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

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

• 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