Transcript Aim # 25

Aim # 25: What is Radioactivity?
H.W#. # 25
Study pp. 891-896
Ans. ques. p. 919 # 11,15,16,18,21
Do Now: Solve problem on handout sheet.
I Review of basic nuclear terms
A. nucleons- sub-atomic particles in the nucleus
(protons and neutrons)
B. mass number- sum of the protons and neutrons in a
nucleus
C. atomic number - number of protons in a nucleus
D. isotopes – atoms with the same at. no., but different
mass nos.
e.g. C-12 and C-14
All elements have naturally occurring isotopes.
Their abundance varies from element to element.
233U
235U
238U
e.g. 1H 2H
1
99.985%
1
.015%
92
92
92
trace
0.7%
99.3%
E. nuclide – a nucleus with a specific number of protons
and neutrons
F. radionuclides – nuclei that are radioactive
G. radioisotopes – atoms containing radionuclides
II Predicting Nuclear Stability
A. Nuclei with 2,8,20,28,50,82,or 126 protons and
neutrons are more stable than those with other
numbers.
B. Nuclei with even numbers of both protons and
neutrons are generally more stable than those with
odd numbers of nucleons.
III Radioactivity – the spontaneous breakdown of atomic
nuclei accompanied by the release of particles and/or
radiant energy.
All isotopes of elements with atomic numbers
greater than 83 (Bi) are radioactive (unstable).
IV Radioactive Decay
A. Types of Radiation
Name of Radiation composition charge (amu) symbol
alpha
He nucleus
+2
4
4He
(α)
2
beta
electron
-1
1/1837 0e
-1
gamma
high energy
photon
0
0
0γ
0
(β-)
positron
electron
capture
positive
electron
+1
neutron
0
1/1837
0e
(β+)
1
1
1n
0
B. Decay Modes
1. alpha decay- at. no. decreases by 2,
mass no. decreases by 4
e.g. 226Ra → 222Rn + 4He
88
86
2
Problem: Identify the missing product in the
following alpha decay scheme.
238U → AX + 4He
92
Z
2
Ans:
234Th
90
Problem: X → 208Pb + 4He
82
What is X?
2
Ans:
212Po
84
2. beta decay- at. no. increases by 1
mass no. remains the same
e.g. 234Th → 234Pa + 0e
90
91
-1
Note: the electron (beta particle) comes from the
nucleus.
1n → 0e + 1p
0
-1
1
Problem: 234Pa → AX + 0e
91
Ans:
Z
Identify X
-1
234U
92
Emissions of alpha and beta particles are often
accompanied by the emission of gamma rays.
e.g. 238U → 234Th + 4He + 20γ
92
90
2
0
3. positron emissionA positron is a particle with the same mass as an
electron, but with a positive charge.
This emission occurs in radioisotopes with a low
neutron to proton ratio.
e.g. 11C → 11B + 0e
6
5
1
The atomic no. decreases by one, and the mass no.
remains the same.
WHAT BECOMES OF THE PROTON?
Note: the proton is converted to a neutron
1p → 1n + 0e
1
0
1
4. electron capture – an inner orbital electron is captured
by the nucleus to form a neutron by combining with a
proton.
e.g. 201Hg + 0e → 201Au + 0γ
80
-1
79
0
Note: the process is accompanied by the emission of
γ radiation.
Problem: Supply the missing particle
38K
19
→ 38Ar + ?
18
Ans:
0e
1
7Be
4
+ 0e (orbital electron) → ?
-1
Ans: 7Li + 0γ
4
0
V Radioactive Decay Series – a series of reactions that
begins with an unstable nucleus, and ends with a stable
one.
238U
234Th
→ 234Pa → 234U
230Th
226Ra
222Rn
218Po
214Pb
→ 214Bi → 214Po
210Pb
→ 210Bi → 210Po
206Pb
atomic no.
82
83
84
86
88
90
91
92
There are 3 such series that occur in nature
238U
→ . . . → 206Pb
235U → . . . → 207Pb
232Th → . . . → 208Pb