Nucleon & Proton Number (AQA A Level Physics)

AZX Notation

• 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

Isotopes

• 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