2. Isotopes • Definition: Atoms of the same element that differ in their # of neutrons; therefore, they have different mass numbers. • Nearly every element has isotopes!

3. Hydrogen has 3 isotopes! Hydrogen is the only element whose isotopes have special names.

4. Representing Isotopes

5. Another Way of Representing Isotopes • Iodine – 128 • Iodine – 127 • Iodine – 126 • 128I 127I 126I • What is iodine’s atomic number? These are the mass numbers!!

6. AVERAGE ATOMIC MASSES • The atomic masses that appear on the periodic table are AVERAGE ATOMIC MASSES. In nature, an element’s isotopes will all be present in some amount. The relative amounts of each isotope is determined for an element. The percentages are then multiplied by the mass # for each isotope, then added together to get the AVERAGE ATOMIC MASS!

7. Calculating the Average atomic mass of an element • The element iodine has three isotopes. In a sample of isotope it is found that 80% is iodine-127, 17% is iodine-126, and 3% is iodine-128. • What is the average atomic mass of iodine? • Change percentages to decimal numbers. 80%  0.80 17%  0.17 3%  0.03

8. 2) Multiply the decimal number by the corresponding mass number. 0.80 x 127 = 0.17 x 126 = 0.03 x 128 = 101.6 21.42 3.84

9. Now add the three numbers together to get the average atomic mass. • 101.6 + 21.42 + 3.84 = 126.86 amu