1. ISOTOPES

2. All matter is made up of elements (e.g. carbon, hydrogen, etc.). The smallest part of an element is called an atom. Atom of different elements contain different numbers of protons. The mass of an atom is almost entirely due to the number of protons and neutrons. Atom Review

3. The Atom The atom consists of two parts: 1. The nucleus which contains: 2. Orbiting electrons. protons neutrons

4. X A Z Mass number Atomic number Element symbol = number of protons + number of neutrons = number of protons Isotopic Notation

5. X A Z A = number of protons + number of neutrons (mass number) Z = number of protons (atomic number) A – Z = number of neutrons Number of neutrons = Mass Number – Atomic Number

6. ISOTOPES  THE NUMBER OF PROTONS FOR A GIVEN TYPE OF ATOM NEVER CHANGES.  THE NUMBER OF NEUTRONS CAN CHANGE.  TWO ATOMS WITH THE SAME NUMBER OF PROTONS BUT DIFFERENT NUMBERS OF NEUTRONS ARE CALLED ISOTOPES.

7. ISOTOPES ISOTOPES ARE ATOMS WITH THE SAME NUMBER OF PROTONS BUT DIFFERENT NUMBER OF NEUTRONS.

8. U 235 92 U 238 92 EXAMPLES OF URANIUM ISOTOPES: A Z Number of protons Number of neutrons A Z Number of protons Number of neutrons

9. U 235 92 U 238 92 EXAMPLES OF URANIUM ISOTOPES: Isotopes of any particular element contain the same number of protons, but different numbers of neutrons. A235 Z92 Number of protons92 Number of neutrons143 A238 Z92 Number of protons92 Number of neutrons146

10. Atomic Weight To calculate the atomic weight (average atomic mass) for a particular element we must consider how much of one isotope of an element exists versus another isotope of the same element. These are the "natural" abundances (% abundance) on earth.

11. To calculate the average atomic weight, each exact atomic weight is multiplied by its percent abundance (expressed as a decimal). Then, add the results together and round off to an appropriate number of significant figures. Rule for Calculating Atomic Weight

12. Example #1: Nitrogen mass numberexact weightpercent abundance 1414.00307499.63 1515.0001080.37 This is the solution for nitrogen: (14.003074) (0.9963) + (15.000108) (0.0037) = 14.007

13. Practice Problem # 1 Calculate the average atomic weight for magnesium: mass numberexact weightpercent abundance 2423.98504278.99 2524.98583710.00 2625.98259311.01

14. Answer # 1 This is the solution for magnesium: (23.985042) (0.7899 ) + (24.985837) (0.1000) + (25.982593) (0.1101)= 24.305