3.2 Isotopes  (Page 2/2)

Isotopes

1. Atom A has 5 protons and 5 neutrons, and atom B has 6 protons and 5 neutrons. These atoms are...
   1. allotropes
   2. isotopes
   3. isomers
   4. atoms of different elements
2. For the sulphur isotopes, ${}_{16}^{32}$ S and ${}_{16}^{34}$ S, give the number of...
   1. protons
   2. nucleons
   3. electrons
   4. neutrons
3. Which of the following are isotopes of ${}_{17}^{35}$ Cl?
   1. ${}_{35}^{17}$ Cl
   2. ${}_{17}^{35}$ Cl
   3. ${}_{17}^{37}$ Cl
4. Which of the following are isotopes of U-235? (X represents an element symbol)
   1. ${}_{92}^{238}$ X
   2. ${}_{90}^{238}$ X
   3. ${}_{92}^{235}$ X

Relative atomic mass

It is important to realise that the atomic mass of isotopes of the same element will be different because they have a different number of nucleons. Chlorine, for example, has two common isotopes which are chlorine-35 and chlorine-37. Chlorine-35 has an atomic mass of 35 u, while chlorine-37 has an atomic mass of 37 u. In the world around us, both of these isotopes occur naturally. It doesn't make sense to say that the element chlorine has an atomic mass of 35 u, or that it has an atomic mass of 37 u. Neither of these are absolutely true since the mass varies depending on the form in which the element occurs. We need to look at how much more common one is than the other in order to calculate the relative atomic mass for the element chlorine. This is the number that you find on the Periodic Table.

Relative atomic mass

Relative atomic mass is the average mass of one atom of all the naturally occurring isotopes of a particular chemical element, expressed in atomic mass units.

Isotopes

1. Complete the table below:

   Isotope	Z	A	Protons	Neutrons	Electrons
   Carbon-12
   Carbon-14
   Chlorine-35
   Chlorine-37

2. If a sample contains 90% carbon-12 and 10% carbon-14, calculate the relative atomic mass of an atom in that sample.
3. If a sample contains 22.5% Cl-37 and 77.5% Cl-35, calculate the relative atomic mass of an atom in that sample.

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. For example, think 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 because of new ideas and discoveries:
  • What were these new ideas?
  • Were they controversial? If so, why?
  • What role (if any) did technology play in developing these new ideas?
  • How have these ideas affected the way we understand the world?
• Many people come up with their own ideas about how the world works. The same is true in science. So how do we, and other scientists, know what to believe and what not to? How do we know when new ideas are 'good' science or 'bad' science? In your groups, discuss some of the things that would need to be done to check whether a new idea or theory was worth listening to, or whether it was not.
• Present your ideas to the rest of the class.