1. Isotopes

2. What is an isotope? Not all atoms of an element are identical. Some elements have isotopes. Isotopes (iso means “same”) are atoms with the same atomic number, but a different mass number. This means that they have the same number of protons and electrons, but different numbers of neutrons.

3. Eg; Hydrogen Most atoms of hydrogen have one proton in the nucleus surrounded by one electron. Two other isotopes exist in small amounts; Deuterium also has one proton and one electron, but in addition there is a neutron. This means that Deuterium still has an atomic number of 1, but has a mass number of 1+1 = 2. Tritium has one proton, one electron, but two neutrons. It has an atomic number of 1, but a mass number 1+2 = 3.

5. Standard Hydrogen DeuteriumTritium Number of protons 111 Number of electrons 111 Number of neutrons 012 Atomic number 111 Mass number 123

6. Chlorine Chlorine has two isotopes. Both have an atomic number of 17. They have 17 protons and 17 electrons. One has a mass number of 35. It has 18 neutrons The other has a mass number of 37. It has a 20 neutrons.

7. 35 Cl 37 Cl Number of protons 17 Number of electrons 17 Number of neutrons 1820 Atomic number 17 Mass number 3537

8. Describe the three isotopes of carbon; 12 C 13 C 14 C. Carbon has an atomic number of 6. This means that all three isotopes will have 6 protons and 6 electrons. N o neutrons = Mass number - Atomic number Therefore in 12 C there will be 12 – 6 = 6 neutrons. In 13 C 13 – 6 = 7. In 14 C 14 – 6 = 8.

9. Effect of isotopes on RAM All terrestrial sources of an element have the isotopes in the same proportions. If isotopes are present only in small amounts their extra mass is not significant. But if a heavy isotope is abundant then its mass has to be taken into account. Hence the RAM should be defined as the average mass of an atom relative to 12 C. This is why some mass numbers are not given as whole numbers.

10. How to use isotopic data to calculate the RAM. Three quarters of terrestrial chlorine is 35 Cl. One quarter is 37 Cl. The mass numbers must be weighted and averaged to calculate the RAM. Eg; (0.75 x 35) + (0.25 x 37) = 35.5