Radioactivity – Particles are emitted from an unstable nuclei.
Name
Alpha
Beta positron gamma symbol
α
β -

β +
2
4
He

0
1 e
1
0 e
High Energy
Photons


11
C

6
11
B
 
5
14
6
C
 14
7
N
 
226
Ra

88
222
Rn

86
31
P

15
30
15
P
 


The time taken for the activity, or the number of atoms remaining, to fall by half is called the half-life
T
1/ 2

Modified Graph lnN vs time


A
 
A


N

N
 t
N

N
0 e
  t
A

A
0 e
  t
A = the activity in becquerels (Bq) at time t
A
0
= the original activity of the sample
ΔN = number of disintegrations
Δt = time interval
N
0
= the original number of atoms in the sample.
N = the number of atoms remaining after time t
λ = the decay constant e = mathematical constant

N

N
0 e
  t

T
1
2
 ln 2



Example: A sample of radioactive isotope originally contains
1.0 x 10 24 atoms and it has a half-life of 6.0 hours. Find: a)The decay constant b)The initial activity c)The number of atoms remaining after 12 hours d)The number of atoms remaining after 30 minutes