Nucleon & Proton Number (AQA A Level Physics)

• A nucleus can be described using  notation

notation is used to describe the constituents of a nucleus

• 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

• Elements are defined by a fixed number of protons in their atoms
  • For example, all hydrogen atoms have 1 proton, and all carbon atoms have 6 protons

• However, atoms of an element can have different numbers of neutrons
  • These different versions of elements are called isotopes

• An isotope is defined as:

Nuclei that have the same number of protons but different numbers of neutrons

• For example, hydrogen has two isotopes, deuterium and tritium
  • All three isotopes contain 1 proton, but different numbers of neutrons

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

• Since nucleon number A includes the number of protons and neutrons, an isotope of an element will have
  • A fixed proton number, Z
  • A different nucleon number, A

• Some isotopes have an imbalance of neutrons and protons which makes them 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

Isotopic Data

• Isotopic data is defined as:

The relative amounts of different isotopes of an element present within a substance

• The mass of an element is often given as relative atomic mass
• The relative atomic mass of an element can be calculated using the relative abundance values
• The percentage abundance of different isotopes in a sample can be obtained using a mass spectrometer

Table of isotopic data for a sample of oxygen

• For example, a sample of oxygen may contain three isotopes: ,  and 
• The relative atomic mass of this sample of oxygen can be calculated using:

(16 × 0.9976) + (17 × 0.0004) + (18 × 0.002) = 16.0044

• To two decimal places, the relative atomic mass of the sample of oxygen is 16.00

• A common use of isotopic data is carbon dating of archaeological artefacts 
  • This involves using the ratio of the amount of stable isotope carbon-12, to the amount of unstable isotope, carbon-14
  • The age of a sample of dead tissue can be determined by comparing the ratio of these isotopes to the ratio in a sample of living tissue

ANSWER: B

Step 1: Properties of isotopes

    Isotopes are nuclei with the same number of protons but different number of neutrons

    The nucleon number is the sum of the protons and neutron

    Therefore, an isotope has a different nucleon number too

Step 2: Calculate protons in the  first nucleus

Nucleon number: 37

Neutrons: 20

Protons = 37 − 20 = 17

Step 3: Calculate protons in the second nucleus

Nucleon number: 35

Neutrons: 18

Protons = 35 − 18 = 17

Step 4: Conclusion

Therefore, they have the same number of protons but different numbers of neutrons and are isotopes of each other

The correct answer is therefore option B