Peluruhan Radioaktif

CHAPTER 22

Nuclear Chemistry II. Radioactive Decay I

II

III IV

(p. 705 - 712)

A. Types of Radiation  Alpha particle (α)  helium nucleus  Beta particle (β-)  electron  Positron (β+)  positron

4 2 0 -1

He 1-

e

 Gamma (γ)  high-energy photon

2+

0 +1

e

paper

lead

1+ 0

concrete

B. Nuclear Decay  Alpha Emission 238 92

parent nuclide

U→

Th + He

234 90

daughter nuclide

4 2

alpha particle

Numbers must balance!!

B. Nuclear Decay  Beta Emission 131 53

I→

131 54

Xe + e 0 -1

electron

 Positron Emission 38 19

K→

38 18

Ar +

0 +1

e positron

B. Nuclear Decay  Electron Capture 106 47

Ag + e → 0 -1

106 46

Pd

electron

 Gamma Emission  Usually follows other types of decay.  Transmutation  One element becomes another.

B. Nuclear Decay  Why nuclides decay…  need stable ratio of neutrons to protons 238 92

U→ I→

131 54

K→

38 18

131 53 38 19

106 47

Th + He

234 90

4 2

Xe + e

Ar +

Ag + e → 0 -1

0 -1

0 +1

106 46

e

Pd

DECAY SERIES TRANSPARENCY

C. Half-life  Half-life (t½)  Time required for half the atoms of a radioactive nuclide to decay.  Shorter half-life = less stable.

C. Half-life

mf = m ( )

1 n i 2

mf: final mass mi: initial mass n: # of half-lives

C. Half-life  Fluorine-21 has a half-life of 5.0 seconds. If you start with 25 g of fluorine-21, how many grams would remain after 60.0 s?

GIVEN:

WORK:

t½ = 5.0 s

mf = mi (½)n

mi = 25 g

mf = (25 g)(0.5)12

mf = ?

mf = 0.0061 g

total time = 60.0 s n = 60.0s ÷ 5.0s =12