TEACHER NOTES – Nuclear Chemistry
6.D - TOPIC 1 – Isotopes & Calculating Average Atomic Mass
1. Example – Potassium (There are 3 different kinds of K atoms.)
Ex. 39 = sum of protons and neutrons; Ex. 93.25% - abundance (how often found in nature)




Potassium – 39 Potassium – 40 Potassium – 41
93.25%
6.73%
0.12%
Protons
19
19
19
Neutrons
20
21
22
Electrons
19
19
19
What are isotopes? – atoms with the same number of protons but different numbers of neutrons
Based on the data in the table, which isotope is heavier? (P-41) Why? (more neutrons)
The discovery of isotopes disproved Dalton’s Theory (Dalton said all atoms of the same element were the
same.)
All atoms of the same element MUST have same # of PROTONS, but not necessarily the same # of
neutrons.
Mass number (protons + neutrons)
39
Two ways to write the symbol for an isotope:
#1. Potassium – 39
or
#2.
19
K
Atomic number (# of protons)
2. Mass Number = PROTONS + NEUTRONS
(The atomic # identifies the # of protons; the mass number identifies the sum of p+ and n0.)
3. Isotope Similarities
a. Chemically and physically alike (same number of valence electrons - determines the chemical behavior)
b. Same # of protons and electrons (atoms - neutral)
Isotope Differences
a. Different # of neutrons
b. Different mass number
c. Different masses
4. Examples: Fill in the chart for the atoms listed below:
Element
Atomic #
Mass #
# Protons
# Neutrons
Neon-22
10
22
10
12
Calcium-46
20
46
20
26
# Electrons
10
20
Symbol
22 Ne
10
same number
46 Ca
20
8
17
26
31
64 Zn
124
80
5. Atomic Mass – The weighted average mass of the ISOTOPES of that element.
1. Expressed in AMU’s
2. AMU = atomic mass unit (1amu = 1/12 the mass of a carbon – 12 atom)
a. proton = 1 amu
b. neutron = 1 amu
c. electron = 1/1840 amu
17
Cl
35.45
6. Usually not a whole number since it is a weighted average of the isotopes.
7. Go back to the table above - Which isotope is more abundant in nature, P-39, P40, or P-41?
Potassium-39 Why? Because the atomic mass of naturally occurring potassium (on PT) is closer to the mass
of P-39 than to the mass of P-40 or P-41.
8. Average Atomic Mass = (% abundance)(mass) + (% abundance)(mass) + .......
Isotope 1
Isotope 2
a. Ex.1. There are 2 isotopes of chlorine. Calculate chlorine’s average atomic mass.
35Cl (75.77% abundance) and
37Cl (24.23%abundance)
b. Ex.2. There are 2 isotopes of copper. Copper-63 has an abundance of 69.1% and Copper-65
has an abundance of 30.9%. Calculate copper’s average atomic mass.
9. Practice – Average Atomic Mass
10. Lab. Coinium (Astroium or Candium)
TOPIC 2 - Discovery of Radioactivity (10-15 min - should NOT be the main focus of the lesson)
Wilhelm Roentgen (1845-1923)
-Studied fluorescent materials that glowed when hit with a beam of electrons.
- Discovered that these materials were emitting rays (radiation) even without electron beam. These rays caused the
photographic plates (film) to darken.
- Called these mysterious rays – X rays.
Antoine Henri Becquerel (1852-1908)
It was the month of February in the year of 1896. Antoine Henri Becquerel, a French scientist, was conducting an
experiment which started with the exposure of a uranium-bearing crystal to sunlight. Once the crystal had sat in the
sunshine for a while, he placed it on a photographic plate. As he had anticipated, the crystal produced its image on the
plate. Becquerel theorized that the absorbed energy of the sun was being released by the uranium in the form of x-rays.
Further testing of this theory had to be put off for a few days because the sky had
clouded up and the sun had disappeared. For the next couple of days he left his
sample of uranium in a closed drawer along with the photographic plate.
When the weather had cleared, he returned to the drawer to retrieve his gear. He was
surprised to find that the crystal had left a clear, strong image on the photographic
plate.
How could this be? There was no source of energy to produce the image! What Becquerel had accidentally discovered
was that a piece of mineral which contained uranium could produce it's image on a photographic plate in the absence of
light. What he had discovered was radioactivity! He attributed this phenomenon to spontaneous emission (emission
that does not require energy) of the uranium.
Although Becquerel did not pursue these findings, it wasn't long before others would.
Pierre Curie (1859-1906) and Marie Curie (1867-1934)
They isolated the component emitting the rays (uranium). The ore also contained two more radioactive elements, radium
and polonium (Nobel Prize).
Radioactivity – process of giving off rays
Radiation – the penetrating rays (gamma) and particles (alpha, beta) emitted by a radioactive source (ex. uranium salt).
Radioisotope Detecting radioactivity
The Geiger counter
Unit of measurement - The unit used to measure radiation dosage is the rem, which stands for roentgen
equivalent in man.
(If time) Sources of Radiation
Govt. recommendations- a person should be exposed to less than 500 millirems= 0.50rems (fatal) of other radiation a year. rem=unit used to
measure radiation; avg annual exposure= 100-300 millirems = 0.10-0.30rems
Ex. dental X ray exposure - harmless does of 0.0005 rem.
0.10-0.30rems :
1. Cosmic radiation - 0.02-0.05rems
2. Radiation from ground – 0.025-0.17rems
Effects -depends on amt. and type of radiation
3. from buildings – 0.01-0.16rems
1. DNA
4. from air – 0.02-0.26 rems
2. Bone marrow
2. Other radiation3. Reproductive organs
 smoking
4. Intestinal lining
 medical treatmen;
 living at high altitudes

x-ray= approx 0.05 rems
(If time) Effects of radiation exposure on human health
Although a dose of just 25 rems causes some detectable changes in blood, doses to near
100 rems usually have no immediate harmful effects. Doses above 100 rems cause the
first signs of radiation sickness including: nausea, vomiting, headache, some loss of white
blood cells.
Doses of 300 rems or more cause temporary hair loss, but also more significant internal
harm, including damage to nerve cells and the cells that line the digestive tract. Severe
loss of white blood cells, which are the body's main defense against infection, makes
radiation victims highly vulnerable to disease. Radiation also reduces production of blood
platelets, which aid blood clotting, so victims of radiation sickness are also vulnerable to hemorrhaging. Half of
all people exposed to 450 rems die, and doses of 800 rems or more are always fatal. Besides the symptoms
mentioned above, these people also suffer from fever and diarrhea. As of yet, there is no effective treatment-so death occurs within two to fourteen days.In time, for survivors, diseases such as leukemia (cancer of the
blood), lung cancer, thyroid cancer (thyroid is most vulnerable gland) , breast cancer, and cancers of
other organs can appear due to the radiation received.