RADIOACTIVITY
Chemist found that after Bismuth elements are not stable, to become more stable they emit some radioactive rays. There are 4 types of radioactive emission: alpha a, beta b, gamma g and n.
For an atom, to show radioactive property, they should obey the following rules:
- The ratio between the numbers of neutrons and protons in a nucleus is greater than 1.5
- The atomic number is bigger than 83.
Exception: C can emit some radioactive rays.
In radioactive reactions following rules are applied:
- On both sides, atomic number must be equal.
- On both sides, the total atomic mass must be equal.
a bX + c d Y ® ac b d Z
Types of Radioactive Rays
1. a rays:a particles are +2 charged, it reduces mass by number 4 and reduces atomic number by 2.
a=24He 2+
92 238 U ® a + 90 234X
2. b rays: are -1 charged particles. b emission causes transition one proton to a neutron.
-1 0 b
81 210 X ® b + 82 210 Y
- g rays: g rays do not have any charge. It just releases energy. g rays can not change neither atomic number nor atomic mass. 00g
- n rays: it doesn’t change the atomic number, it reduces atomic mass by number 1.
0 1n
Ex: 80 275X® 3a + A ® 3b + B ® 4g + C find A, B, C.
Ex: 74 263X ® 4a + 2n + 3b + 4g + Y find Y.
Half Life
For radioactive elements the time needed to decay half of initial amount of the element is called half life.
m=m0/2n m0 = initial mass
m= mass left
n=t/t1/2 t= time passed
t1/2 = half-life
Ex: The half life of 214-83 Bi is 1 hour. What mass of 64g of sample will remain undecayed after 4 hours? (4g)
Ex: 75% of a radioactive element decays in 20 hours. What is half life? (10h)
Ex: The half life of Pb(212) is 11 hours. After 33 hours what mass of Pb remains if we have 2 mole initially? (53 g)