1. ISOTOPES & AVERAGE ATOMIC MASS

2. Agenda P. 26-27-Average Atomic Mass (O/S) HW:
P.27# 1; P.29 # 1-9 and w/s

3. ATOMS Each elements is made up unique atoms
The atoms of an element have identical chemical properties
All atoms of an element may have the SAME NUMBER OF ELECTRONS AND PROTONS
Some atoms of an element may may have a different number of NEUTRONS and therefore different ATOMIC MASS NUMBER
Isotopes of the same element have the same chemical properties but different physical properties ( different mass).

4. ISOTOPES
Atoms of the same element having the same number of protons in their nucleus but a different number of neutrons are isotopes of each other
The average atomic mass number shown in the periodic table is the result of an average mass based on the abundance of each isotope.
Most elements occur naturally as mixtures of isotopes, as indicated on your handout. The percentage of each isotope in the naturally occurring element on Earth is nearly always the same, no matter where the element is found.

5. AVERAGE ATOMIC MASS
Isotopes with different mass numbers exist in a fixed ratio in a sample of an element
The percent abundance of each isotope can be determined by mass spectroscopy
This percent abundance is used to calculate the average atomic mass of the element
It is used as a weighted measure of the mass of a specific isotope
AVERAGE ATOMIC MASS [A and B are isotope masses]
AAM = % abundance x mass A + % abundance x mass B

6. AVERAGE ATOMIC MASS Example:
A sample of carbon has two isotopes C-12 and C-13, with C-12 comprising % of the sample and C-13 comprising 1.11 % . Find the average atomic mass.
A.A.M. = (0.9889)(12) (0.0111)(13) =
= a.m.u.
Therefore the average atomic mass of carbon is u.

7. Q- Sometimes an isotope is written without its atomic number - e. g
Q- Sometimes an isotope is written without its atomic number - e.g. 35S (or S-35). Why?
A- The atomic # of an element does not change. Although the number of neutrons can vary, atoms have definite numbers of protons.
Example: A sample of two naturally occurring isotopes of lithium, Li-6 and Li-7 have masses of 6 u and 7 u, respectively. Which of these two occurs in greater abundance?
AMM = u ( as shown on the periodic table)
Lithium-7 must be more abundant

8. 6Li Li
3 p+
3 n0
2e– 1e–
3 p+
4 n0
2e– 1e–
Every element on the periodic table has at least 2 isotopes and some elements have as many as 25 isotopes.

9. The isotopes of hydrogen have separate names rather than being called hydrogen-1, hydrogen-2, etc. Their names are protium (H-1), deuterium (H-2), and tritium (H-3).

10. Agenda P. 23- 29 Radioisotopes Half Life Definition & C-14 dating
HW: Worksheets

11. Radioisotopes
Some isotopes are stable like H-2 while other isotopes are unstable like C-14
Unstable isotopes are called radioisotopes
Radioisotopes undergo radioactive decay resulting in the production of ionizing radiation and a more stable nucleus
Each radioisotope has a characteristic rate of decay that is known as a half-life
Can you give me a number of uses of radioactivity
To kill bacteria
Medicine cancer killing radation Co-6o , Rd- 126
Using Einstein’s equation E=mc2, we can actually calculate the energy that was formed when the
nucleus was formed! This is called the nuclear binding energy. (It can also be thought of as the amount of energy required to break apart the nucleus; therefore, the nuclear binding energy is also a measure of the stability of a nucleus.)

12. HALF LIFE
Radioisotopes undergo radioactive decay at fixed unique rates that are characteristic for each different radioisotope
The time it takes for half (1\2) of the nuclei in a radioactive sample to decay is known as the half-life of the radioisotope
Half-lives may vary from a few seconds to many years [Po-226 has a 0.16 s half-life while Cs-142 has a 5 x 1015 a (years) half-life]

13. Carbon dating Uses C-14 to date organic material.
Carbon- 14 has a half-life of 5730 a.
Carbon is constantly recycled through the carbon cycle through living organisms, and the proportion of carbon - 14 remains constant. Once an organism dies, the recycling of carbon stops and C -14 starts to decay. As time goes on the remains contain fewer and fewer C -14 atoms.
By comparing the amount of C -14 in an organic sample to the amount present in living organisms, it is possible to determine the age of the organic sample.

14. How carbon -14 is made.

15. Uses of radioisotopes
Non living material such as rocks can be dated similarly using K -40 with a half-life of 1.3 x 109 a.
Anthropologists and geologists commonly use these techniques to date both once living artifacts as well as rocks.
Other disciplines that find these techniques useful are forensic pathologists, museum curators, art experts and art authenticators.

16. Half Life Problems Amount of Phosphorus-32 Time Elapsed 4mg
Example Problem: Phosphorus-32 has a half-life of 14.3 days. How many mg of phosphorus-32 remain after 57.2 days if you start with 4.0 mg of the isotope?
Amount of Phosphorus-32
Time Elapsed
4mg
0 days have past
2mg
14.3 days have past
1mg
28.6 days have past
0.5mg
42.9 days have past
0.25mg
57.2 days have past

17. Another way to work problems:
# of half-lives = time elapsed x ratio for half-life
# of half-lives = 57.2 days x 1 half-life = 4 half-lives
14.3 days
So now we know the answer will be:
4mg x ½ x ½ x ½ x ½ = 0.25mg or
4mg x (½)4 = 0.25mg

18. Another way to work problems: Use the following formula
Af is the amount of substance left
Ai is the original amount of substance
t is the elapsed time
t1/2 is the half-life of the substance

19. Radioisotopes
Unstable isotopes undergo radioactive decay giving off radiation and changing the composition of their nuclei. This emission of radiation from the nucleus of an atom is known as RADIOACTIVITY. There are 3 types of radiation given off by radioisotopes. 1. ALPHA particles (42He2+ , ) - the nuclei of helium atom - have 2 protons and 2 neutrons - have a 2 + charge - stopped n a few centimeters of air

20. electrons travelling at a very high speed have a negative charge
2. BETA particles ( , e- )
electrons travelling at a very high speed
have a negative charge
stopped by a thin sheet of lead or aluminum foil.
3. GAMMA rays ()
do not consist of particles
a form of high energy radiation, similar to X-rays
stopped by a sheet of lead several centimeters thick or reinforced concrete.
gamma rays () are high-energy electromagnetic waves emitted from a nucleus as it changes from an excited state to a ground energy state. Very similar to light, but is much more dangerous. Gamma emission usually occurs immediately following other types of decay