1. Isotopes and Average Atomic Mass Chemistry 11- Chapter 1 Section 4

2. Isotopes  The number of protons define the type of atom: 6 protons is carbon 1 proton is hydrogen 8 protons is oxygen 92 protons is uranium  However the number of neutrons can vary within a given type of atom

3. Trend in the number of neutrons as atomic number increases

4. Hydrogen Isotopes  There are three main isotopes of hydrogen Hydrogen – 1p, 0n, 1e Deuterium – 1p, 1n, 1e Tritium – 1p, 2n, 1e

5. Relative Mass of an Atom  The mass of an atom is expressed in atomic mass units (u)  In the past, hydrogen was used as a standard of 1 u but because of the isotopes, the standard was changed to carbon  Atomic mass is determined based upon carbon- 12 as a standard with a mass exactly equal to 12u. One twelve of C-12 is equal to 1u  Using this, the relative mass of every atom on the periodic table can be determined (i.e. oxygen 16 would have a mass of 16u)

6. Isotopic Abundance  Because elements as a combination of different isotopes, it is useful to consider the isotopic abundance  For example, magnesium is composed of: Magnesium-24 – 79% Magnesium-25 – 10% Magnesium-26 – 11%  Regardless of where you find magnesium, it will be composed of these percentages

7. Mass spectrometer  Highly specialized piece equipment that can identify the mass of an element or compound in a mixture and its relative abundance  It directs the element in gas form across a tube and the beam is deflected by a magnetic field…heavy elements keep going straight, light elements veer of quickly. The detector plate senses the elements as they hit it.

9. Isotopes and radioisotopes  Some isotopes are stable but not all  All elements after Bismuth are unstable  Unstable isotopes break down- the nucleus, and transmutate to new elements and give off radiation in the process.  They are described as radioactive and break down in a predictable way called half life  There are many uses for radioisotopes especially in the medical and research field

10. Nuclear radiation 3 types: alpha, beta and gamma  Alpha= heavy, positive helium nucleus  = 2 P and 2 N, can be stopped by paper  Beta= negatively charged electron  = can be stopped by aluminum or plexiglass (thick see-through plastic)  Gamma= high energy electromagnetic radiation  = no mass or charge, can be stopped by thick lead

12. Average Atomic Mass and the Periodic Table  The average atomic mass that appears on the periodic table is the average of the mass of all the element’s isotopes  Therefore, even carbon, which is used as the standard, does not have an average atomic mass of 12 but rather 12.01 due to the presence of carbon-14  Carbon-14 is about 1.1% of all carbon while carbon-12 is the remaining 98.1%

13. Average Atomic Mass Calculation

14. Average Atomic Mass  Naturally occuring silver exists as two isotopes. From the mass of each isotope and the isotopic abundance listed below, calculate the average atomic mass of silver IsotopeAtomic mass (u)Abundance (%) Silver-107106.951.8 Silver-109108.948.2

15. Average Atomic Mass

16. You try  1. Chlorine has two naturally occurring isotopes. One has a mass of 34.969 and an abundance of 75.77% and the other a mass of 36.966 and abundance of 24.34%. Calculate the average atomic mass.  2. Calculate the average atomic mass of oxygen given the three isotopes abundance are 99.759%, 0.037%, 0.204% for the masses 15.995, 16.995 and 17.999 respectively.

17.  1. 35.494  2. 15.999