Radioactive Decay (CIE IGCSE Physics)

EXTENDED

• The most stable nuclei have roughly the same number of protons as neutrons
  • If there were too many protons, then the repulsive force created by them all having the same positive charge, would cause the nucleus to repel when it becomes very large
• Therefore, if a nucleus has an imbalance of protons or neutrons, it is more likely to decay into small nuclei until it gets to a stable nucleus with roughly the same number of each
• Therefore, Isotopes of an element may be radioactive due to:
  • An excess of protons or neutrons in the nucleus
  • The nucleus being too heavy
• An example of this is the isotope of hydrogen–1

Hydrogen Isotopes

• H-1 is the stable nucleus of hydrogen
  • H-2 (deuterium) adds on one more neutron
  • H-3 (tritium) adds on another neutron, making 2 neutrons to 1 proton. This is much more unstable than H-1 or H-2

• If an nucleus is too heavy, this means it has too many protons and neutrons
  • The forces in the nucleus will be weaker in keeping the protons and neutrons together
  • This can also cause the nucleus to decay
• An example of this is Uranium–238 which is used in nuclear fission
  • This nucleus has 238 protons and neutrons
• The decay of Uranium–238 gradually reduces the mass number of the element which it decays into
  • This is done through alpha (α) or beta (β) decay

Uranium–238 Decay Chain

• During α-decay or β-decay, the nucleus changes to a different element
• The initial nucleus is often called the parent nucleus
• The nucleus of the new element is often called the daughter nucleus

Alpha decay changes a parent nucleus into a daughter nucleus of a new element

• The daughter nucleus is a new element because it has a different proton and/or nucleon number to the original parent nucleus
• This can be seen on a graph of N (neutron number) against Z (proton number)

Graph of N against Z for the decay of Pu–239

• When Pu-239 decays by alpha to U-235, it loses 2 protons and 2 neutrons
  
  • U (Uranium) is a completely different element to Pu (Plutonium)

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• A nucleus decays to increase its stability by reducing the number of excess neutrons
  
  • This is done by alpha or beta decay
• If the nucleus has too much energy, this is given off in the form of radiation
  • This is often gamma radiation

Alpha Decay

• During alpha decay an alpha particle is emitted from an unstable nucleus
• A completely new element is formed in the process

Alpha decay usually happens in large unstable nuclei, causing the overall mass and charge of the nucleus to decrease

• An alpha particle is a helium nucleus
  • It is made of 2 protons and 2 neutrons

• When the alpha particle is emitted from the unstable nucleus, the mass number and atomic number of the nucleus changes
  • The mass number decreases by 4
  • The atomic number decreases by 2

• The charge on the nucleus also decreases by 2
  • This is because protons have a charge of +1 each

Beta Decay

• During beta decay, a neutron changes into a proton and an electron
  • The electron is emitted and the proton remains in the nuclei

• A completely new element is formed because the atomic number changes

Beta decay often happens in unstable nuclei that have too many neutrons. The mass number stays the same, but the atomic number increases by one

• A beta particle is a high-speed electron
• It has a mass number of 0
  • This is because the electron has a negligible mass, compared to neutrons and protons

• Therefore, the mass number of the decaying nuclei remains the same
• Electrons have an atomic number of -1
  • This means that the new nuclei will increase its atomic number by 1 in order to maintain the overall atomic number before and after the decay

• The following equation shows carbon-14 undergoing beta decay
  • It forms nitrogen-14 and a beta particle
  • Beta particles are written as an electron in this equation

Gamma Decay

• During gamma decay, a gamma ray is emitted from an unstable nucleus
• The process that makes the nucleus less energetic but does not change its structure

Gamma decay does not affect the mass number or the atomic number of the radioactive nucleus, but it does reduce the energy of the nucleus

• The gamma ray that is emitted has a lot of energy, but no mass or charge

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• Radioactive decay events can be shown using a decay equation
• A decay equation is similar to a chemical reaction equation
  • The particles present before the decay are shown before the arrow
  • The particles produced in the decay are shown after the arrow

• During decay equations the sum of the mass and atomic numbers before the reaction must be the same as the sum of the mass and atomic numbers after the reaction

• The following decay equation shows Polonium-212 undergoing alpha decay
  • It forms Lead-208 and an alpha particle
  • An alpha particle can also be written as a helium nucleus (Symbol He)

The polonium nucleus emits an alpha particle, causing its mass and charge to decrease. This means it changes into a new element

Alpha Decay Equation

• When the alpha particle is emitted from the unstable nucleus, the mass number and atomic number of the nucleus changes
  • The mass number decreases by 4
  • The atomic number decreases by 2

Alpha decay equation

Beta Decay Equation

• During beta decay, a neutron changes into a proton and an electron
  • The electron is emitted and the proton remains in the nuclei

Beta decay equation

Gamma Decay

• The gamma ray that is emitted has a lot of energy, but no mass or charge
• Here is an example of Uranium-238 undergoing gamma decay
  • Notice that the mass number and atomic number of the unstable nuclei remains the same during the decay

Gamma decay equation

ANSWER:   A

Step 1: Calculate the mass number of the original nucleus

• • The mass number is equal to the number of protons plus the number of neutrons
  • The original nucleus has 84 protons and 126 neutrons

84 + 126 = 210

• • The mass number of the original nucleus is 210

Step 2: Calculate the new atomic number

• • The alpha particle emitted is made of two protons and two neutrons
  • Protons have an atomic number of 1, and neutrons have an atomic number of 0
  • Removing two protons and two neutrons will reduce the atomic number by 2

84 – 2 = 82

• • The new nucleus has an atomic number of 82

Step 3: Calculate the new mass number

• • Protons and neutrons both have a mass number of 1
  • Removing two protons and two neutrons will reduce the mass number by 4

210 – 4 = 206

• • The new nucleus has a mass number of 206

ANSWER:  D

Step 1: Calculate the mass number of the original nucleus

• • The mass number is equal to the number of protons plus the number of neutrons
  • The original nucleus has 11 protons and 13 neutrons

11 + 13 = 24

• • The mass number of the original nucleus is 24

Step 2: Calculate the new atomic number

• • During beta decay a neutron changes into a proton and an electron
  • The electron is emitted as a beta particle
  • The neutron has an atomic number of 0 and the proton has an atomic number of 1
  • So the atomic number increases by 1

11 + 1 = 12

• • The new nucleus has an atomic number of 12

Step 3: Calculate the new mass number

• • Protons and neutrons both have a mass number of 1
  • Changing a neutron to a proton will not affect the mass number
  • The new nucleus has a mass number of 24 (the same as before)