Isotopes & Radioactive Decay (HL IB Physics)

• 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 defined as:

An atom (of the same element) that has an equal number of protons but a different number of neutrons

• The element hydrogen has two other isotopes: deuterium and tritium

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

• 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
• Some 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

Differences between isotopes

• The number of neutrons in an atom does not affect the chemical properties of an atom, such as its charge, but only its mass
  • This is because neutrons have no charge but do have mass

• The charge of the nucleus of a particular element is always the same
• In the periodic table, the mass number of Chlorine is often given as 35.5

This section of the periodic table shows chlorine as having a mass number of 35.5, but other elements have an integer mass number

• The mass number of Chlorine is given as 35.5 because it has two isotopes with mass numbers of 35 and 37, and these occur in a ratio of 3:1 respectively so 35.5 is the average nucleon / mass number
• The number of electrons and protons in different isotopes remains the same
• Less common isotopes tend to be more unstable due to the imbalance of protons and neutrons

Isotopic Data

• Isotopic data is defined as: 

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

• It is used to identify an isotopic signature within organic and inorganic materials
• Isotopic data is often used for determining the age of archaeological findings and is used in radioactive dating
• Carbon–14 is a naturally occurring isotope most often used for this since it is present in all living beings and undergoes radioactive decay
• When a plant or animal dies, the natural radioactive decay of this isotope means the concentration of the carbon–14 in its tissue gradually reduces
• Since carbon–14 has a long half-life of around 6000 years, the half-life can be used to determine the age of the plant or animal when it died

• Radioactive decay is defined as:

The spontaneous disintegration of a nucleus to form a more stable nucleus, resulting in the emission of an alpha, beta or gamma particle

• The random nature of radioactive decay can be demonstrated by observing the count rate of a Geiger-Muller (GM) tube
  • When a GM tube is placed near a radioactive source, the counts are found to be irregular and cannot be predicted
  • Each count represents a decay of an unstable nucleus

• These fluctuations in count rate on the GM tube provide evidence for the randomness of radioactive decay

The variation of count rate over time of a sample radioactive gas. The fluctuations show the randomness of radioactive decay

Characteristics of Radioactive Decay

• Radioactive decay is both spontaneous and random
• A spontaneous process is defined as:

A process which cannot be influenced by environmental factors

• This means radioactive decay cannot be affected by environmental factors such as:
  • Temperature
  • Pressure
  • Chemical conditions

• A random process is defined as:

A process in which the exact time of decay of a nucleus cannot be predicted

• Instead, the nucleus has a constant probability, ie. the same chance, of decaying in a given time
• Therefore, with large numbers of nuclei, it is possible to statistically predict the behavior of the entire group