Transcript Nuclear Chemistry Introduction Isotopes

Nuclear Chemistry Introduction
Isotopes
atomic number (Z)
mass number (A)
mass number (A)
atomic number (Z)
=
the number of protons
=
the number of protons
+
the number of neutrons
12
6
C
number of protons
number of neutrons
6
12 – 6 = 6
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Nuclear Chemistry Introduction
Isotopes
Isotopes are atoms of the same element having a
different number of neutrons.
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Nuclear Chemistry Introduction
Isotopes
• A radioactive isotope, called a radioisotope, is
unstable and spontaneously emits energy to form
a more stable nucleus.
• Radioactivity is the nuclear radiation emitted by a
radioactive isotope.
• Of the known isotopes of all elements, 264 are
stable and 300 are naturally occurring but unstable.
• An even larger number of radioactive isotopes,
called artificial isotopes, have been produced in
the laboratory.
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NUCLEAR RADIATION
Used to :
treat medical conditions
detect and treat tumors
detect the function of an organ
produce scans/images of organs
ORIGIN
Most element have at least one unstable isotopes.
Unstable isotopes arise when the attraction/repulsion
between protons and neutrons in the nucleus is unbalanced.
An unstable nucleus has too few or too many neutrons as
compared to protons.
e.g. 2412Mg
23 Mg
12
(Mg-23)
27 Mg
12
(Mg-27)
RADIOACTIVITY
*
This is emission of small particles of energy or rays
(radiation) by unstable elements (to be stable).
*
Elements that emit radiation spontaneously (e.g.
Thorium, Th and Uranium, U) are said to be radioactive
and called radioisotope.
*
The radiation is emitted from the nucleus of the atom
TYPES OF RADIATION
*
Particles and Rays
• Alpha and Beta Particles and Gamma Rays
1.
ALPHA PARTICLES (α particles)
*
Attracted
towards
negative
field
(positively
charged).
*
Made up of (two protons and two neutrons),
helium nuclei, therefore have charge of +2.
*
Have low penetrating power and are harmless
when they strike the body.
*
Harmful when inhaled or get into body through
open wound and can cause damage to internal
organs.
2.
BETA PARTICLES ( β particles)
*
Attracted towards positive field (negatively charged)
*
Made up of electrons, therefore have a charge of -1
*
Produced
in
the
nucleus
by
transformation
of
neutron into a proton and an electron.
*
Slightly more power than the alpha particles but can
be stopped by heavy clothing.
*
Harmful when inhaled or get into body through open
wound and can cause damage to internal organs.
3.
GAMMA RAYS (γ rays)
*
Not affected by electrostatic field, therefore have no
charge
*
Very penetrating, pass through body and can cause damage
to internal organs and cells as they pass through the body.
*
Used to treat inaccessible brain tumors.
PROTECTION
1.
DISTANCE
*
Increasing the distance between the source of
radiation and the body reduces the effect of the
radiation.
Radiation
=
1
(distance)2
2.
SHIELDING
*
Protective gloves, clothing and tongs must be
used at all times when handling radioactive
material.
3.
LIMITING EXPOSURE (TIME)
*
Limit the time that one spends next to
radioactive material. The shorter the time, the
lesser the dose of radiation.
NUCLEAR REACTIONS
*
Process by which an unstable radioactive nucleus emits
radiation, forming a new nucleus of different composition
*
This result in change of either atomic number, mass number or
both on the new nucleus.
*
The following can be emitted during nuclear reactions;

Alpha particles, α, (42He)

Beta particles, β, (o-1e)

Gamma ray (γ)

Neutron (10n)

Proton (11p) or (11H)

Positron (o1e)
e.g.
1.
A thorium atom (23490Th) decays by alpha
emission.
234 Th
90
2.
→ α + X
234 Th)→ 4 He
90
2
+
230 X
88
234 Th→ 4 He
90
2
+
230 Ra
88
A thorium atom (23490Th) decays by beta and
gamma emission
234 Th
90
→ β + γ + X
234 Th
90
→
o
-1e
234 Th→ o e
90
-1
234 Th
90
→
o
-1e
+ γ + X
+ γ +
+ γ +
234 X
91
234 Pa
91
• Units Of Radiation
• Radiation measured using the Geiger Counter.

Curie. This
indicates the
number
of
nuclear
disintegrations occurring per second from 1g of a radioactive
substance (radium)
1 curie = 3.7 x 1010 disintegrations per second

Becqurel (Bq) = 1 disintegration per second (SI unit)

Rad (radiation absorbed dose) = amount of radiation dosage
absorbed by one gram of substance (human tissue)
Unit gray (Gy)
Rem (radiation equivalent in man) measures biological
damage or damage to body tissues or organs.
LD50 (lethal dose that can cause death to 50 % of the population).
LD50 (dogs) = 300 rem
LD50 (humans) = 500 rem
LD50 (rats) = 800 rem

• HALF-LIFE (t1/2)
• The amount of time required for half the amount of a radioactive
substance to decay.
• Remianing Mass = Mi (1/2)n
• e.g. A 400 mg sample of iodine-131 is left in a laboratory cupboard.
How much of the sample will be left after 80 days if the half-life of
iodine is 8 days.
• NB: For medicinal purposes only radioisotopes with short halflives are used to treat patients.
• e.g. Technetium (6 hours), I-131 (8 days), Gold (2.7 days)
• C-14 (5730 yrs) Used for carbon dating.
NUCLEAR ENERGY
Energy generated from the nucleus of radioactive substances.
NUCLEAR FISSION
Splitting of heavy elements into smaller pieces, accompanied
by large amounts of energy.
236 U
90
+
1 n
0
→
91 Kr
36
+
142
1 n + energy (E = mc2)
Ba
+
3
56
0
Chain reaction (The three neutron produced can bombard
other nuclei to produce more energy)
e.g.
Atomic bomb
NUCLEAR FUSION
Small nuclei combine to form larger nuclei.
2 H
1
e.g.
+
3 H
1
→
Sun
4 He
2
+
1 n,
0
+ energy (E = mc2)
• Write a balanced equation for the following
nuclear reactions;
a. When two oxygen-16 atoms collide, one of the
products is an alpha particle.
b. When californium-249 is bombarded by oxygen18, a new isotope and four neutrons are
produced.
c. Silicon-26 decays by positron emission.
d. Rb-77 decays by beta and alpha emission.
• Cesium-137 has a half-life of 30 years
a. Write a nuclear equation for beta decay of
cesium-137
b. How many grams of a 16 g sample would remain
after 90 years.
c. How many years would be needed for 28 g of
cesium-137 to decay to 3.5g?