# Worked example 4 the relative atomic mass of an

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Worked Example 4: The relative atomic mass of an isotopic element Question: The element chlorine has two isotopes, chlorine-35 and chlorine-37. The abundance of these isotopes when they occur naturally is 75% chlorine-35 and 25% chlorine-37. Calculate the average relative atomic mass for chlorine. Answer Step 1 : Calculate the mass contribution of chlorine-35 to the average rela- tive atomic mass Contribution of Cl-35 = (75/100 x 35) = 26.25 u Step 2 : Calculate the contribution of chlorine-37 to the average relative atomic mass Contribution of Cl-37 = (25/100 x 37) = 9.25 u Step 3 : Add the two values to arrive at the average relative atomic mass of chlorine Relative atomic mass of chlorine = 26.25 u + 9.25 u = 35.5 u. If you look on the periodic table, the average relative atomic mass for chlorine is 35,5 u. You will notice that for many elements, the relative atomic mass that is shown is not a whole number. You should now understand that this number is the average relative atomic mass for those elements that have naturally occurring isotopes. Exercise: Isotopes You are given a sample that contains carbon-12 and carbon-14. 1. Complete the table below: 45

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3.6 CHAPTER 3. THE ATOM - GRADE 10 Isotope Z A Protons Neutrons Electrons Carbon-12 Carbon-14 Chlorine-35 Chlorine-37 2. If the sample you have contains 90% carbon-12 and 10% carbon-14, calculate the relative atomic mass of an atom in that sample. 3. In another sample, you have 22.5% Cl-37 and 77.5% Cl-35. Calculate the relative atomic mass of an atom in that sample. Activity :: Group Discussion : The changing nature of scientific knowledge Scientific knowledge is not static: it changes and evolves over time as scientists build on the ideas of others to come up with revised (and often improved) theories and ideas. In this chapter for example, we saw how peoples’ understanding of atomic structure changed as more information was gathered about the atom. There are many more examples like this one in the field of science. Think for example, about our knowledge of the solar system and the origin of the universe, or about the particle and wave nature of light. Often, these changes in scientific thinking can be very controversial because they disturb what people have come to know and accept. It is important that we realise that what we know now about science may also change. An important part of being a scientist is to be a critical thinker . This means that you need to question information that you are given and decide whether it is accurate and whether it can be accepted as true. At the same time, you need to learn to be open to new ideas and not to become stuck in what you believe is right... there might just be something new waiting around the corner that you have not thought about! In groups of 4-5, discuss the following questions: Think about some other examples where scientific knowledge has changed be- cause of new ideas and discoveries: What were these new ideas?
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