1. ISOTOPES

2. Atom Review
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.

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

4. X A Z Isotopic Notation Mass number
= number of protons + number of neutrons A X
Element symbol Z
Atomic number
= 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 U 235 92 238 92 EXAMPLES OF URANIUM ISOTOPES: A Z Number of protons
Number of neutrons A Z
Number of protons
Number of neutrons

9. U U 235 92 238 92 EXAMPLES OF URANIUM ISOTOPES: A 235 Z 92
Number of protons
Number of neutrons
143 A
238 Z 92
Number of protons
Number of neutrons
146
Isotopes of any particular element contain the same number of protons, but different numbers of neutrons.

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. Rule for Calculating Atomic Weight
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.

12. This is the solution for nitrogen:
Example #1: Nitrogen
mass number
exact weight
percent abundance 14
99.63 15
0.37
This is the solution for nitrogen:
( ) (0.9963) + ( ) (0.0037) =

13. Practice Problem # 1
Calculate the average atomic weight for magnesium:
mass number
exact weight
percent abundance 24
78.99 25
10.00 26
11.01

14. This is the solution for magnesium:
Answer # 1
This is the solution for magnesium:
( ) ( ) + ( ) (0.1000) + ( ) (0.1101)=