1. Neutrons and Isotopes

2. NEUTRONS AND ISOTOPES Unlike the number of protons, the neutrons in the nucleus of atoms of the same element can vary. Atoms of the same element with different numbers of neutrons are called isotopes. 2

3. ISOTOPIC NOTATION Using isotopic notation we can distinguish between isotopes of the same element and calculate the number of neutrons within each atom. 3

4. ISOTOPE STABILITY There are over 400 stable isotopes. Elements of increasing atomic number generally have increasing numbers of isotopes. A stable isotope has balanced the attraction between protons and neutrons with the repulsion of protons to other protons. 4

5. RADIOISOTOPES Radioisotopes are unstable isotopes in which the protons and neutrons are not balanced. Radioisotopes undergo decay when a neutron splits into a proton and an electron which is ejected from the nucleus as a beta particle. 5

6. ISOTOPIC ABUNDANCE Almost every element has naturally occurring isotopes. The percentage of each isotope that occurs in nature is a constant value known as the isotopic abundance. 6

7. MASS SPECTROMETER Scientists distinguish between isotopes using a mass spectrometer. 7

8. PROPERTIES Isotopes of the same element have identical chemical properties but different physical properties. H 2 O b.p. is 100°C 2 H 2 O b.p. is 104°C 8

9. ATOMIC MASS A scale designed for atoms gives their small atomic masses in atomic mass units (amu) An atom of 12 C was assigned an exact mass of 12.00 amu Relative masses of all other atoms was determined by comparing each to the mass of 12 C An atom twice as heavy has a mass of 24.00 amu. An atom half as heavy is 6.00 amu. 9

10. AVERAGE ATOMIC MASS Listed on the periodic table Gives the mass of “average” atom of each element compared to 12 C Average atom based on all the isotopes and their abundance % Calculated as a weighted average Atomic mass is not a whole number 10

11. Isotopes Mass of Isotope Abundance 24 Mg =24.0 amu 78.70% 25 Mg = 25.0 amu 10.13% 26 Mg = 26.0 amu 11.17% Atomic mass (average mass) Mg = 24.3 amu 11 Mg 24.3

12. CALCULATING AVERAGE ATOMIC MASS To calculate average atomic mass of an element… 12

13. Gallium is a metallic element found in small lasers used in compact disc players. In a sample of gallium, there is 60.2% of gallium-69 (68.9 amu) atoms and 39.8% of gallium-71 (70.9 amu) atoms. What is the average atomic mass of gallium ? 13 EXAMPLE

14. 14 SOLUTION

15. USES OF RADIOISOTOPES 15