Notes: Rate of Radioactive Decay

RATE OF RADIOACTIVE
DECAY
NUCLEAR CHEMISTRY

GOALS
• To understand the relationship between half-life, rate of
radioactivity and number of nuclei

REVIEW: RADIOACTIVE DECAY
• When an unstable nucleus decays, it emits high energy
particles or gamma radiation by a process called
radioactive decay

REVIEW: RADIOACTIVE DECAY
• Alpha decay
• A nucleus ejects an alpha particle ( )
• A new element is produced.
• The daughter has a mass 4 less than the parent.
• The daughter has an atomic number 2 less than the parent.
• Example
226 222 4
88 Ra ! 86 Rn + 2 He
4
2 He

REVIEW: RADIOACTIVE DECAY
• Beta Decay
• A nucleus ejects a beta particle ( )
• A new element is produced
• The daughter has the same mass as the parent
• The daughter has an atomic number 1 more than the parent.
• Example
228 228 0
88 Ra ! 89 Ac + "1e
0
!1e

REVIEW: RADIOACTIVE DECAY
• Gamma Decay
• A high energy nucleus emits electromagnetic radiation in the form
of gamma rays
• Gamma rays have no mass and no charge
• Example
234 234 0
90Th
! 90Th
+ 0!

RATE OF RADIOACTIVE DECAY
• Each radioisotope decays at a different rate
• This rate is called the half-life
• It is unique and constant for each radioisotope.

HALF-LIFE
• Half-life
• The time needed for half of the radioactive atoms in a sample to
decay to stable nuclei

LET’S COUNT!

HALF-LIFE

HALF-LIFE
NO OF HALF-LIVES AMT OF PARENT
REMAINING
FRACTION OF PARENT
REMAINING
0 16 100% or 1 (16 ÷ 16)
1 8 50% or ½ (8 ÷ 16)
2 4 25% or ¼ (4 ÷ 16)
3 2 12.5% or ⅛ (2 ÷ 16)

HALF-LIFE

HALF-LIFE

HALF-LIFE

HALF-LIFE
• Can be found by measuring the rate of decay using a
radiation detector

HALF-LIFE

HALF-LIFE
• As parent decays to daughter, you have less unstable
nuclei and more stable nuclei
• The less stable the element, the higher the rate of radiation
• The more radioactive the element, the shorter the half-life