Alpha Decay (AQA GCSE Physics: Combined Science)

• 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

Decay Equations

• The process of alpha decay can be shown as 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 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

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