Isotopes and Relative Atomic Mass
Introduction

You might have heard the term 'radioactive isotopes' mentioned on the news or by people worried about the environment. Sometimes we use radioactive isotopes to help treat cancer. But what is an isotope? In this unit you will find out what an isotope is. We will also see how to calculate the relative atomic mass of atoms, which measures how heavy atoms are relative to one another.

Isotopes
We saw in the unit Atomic Shorthand that each chemical
element
An element is a substance made from only one type of atom. An element cannot be broken down into any simpler substances.
element
has its own unique
atomic number
The atomic number is the number of protons in an atom (which equals the number of electrons).
atomic number
. That means that all the atoms of any particular
element
An element is a substance made from only one type of atom. An element cannot be broken down into any simpler substances.
element
must have the same number of protons (and therefore electrons). So any
atom
An atom is the smallest particle of an element that can still be defined as that element.
atom
of hydrogen has one proton, otherwise it wouldn't be hydrogen! If the atom had two protons, it would be helium.


But there is no reason why the number of neutrons can't vary. In fact, there are three types of hydrogen atom, each containing a different number of neutrons. We call these isotopes of hydrogen. So isotopes are atoms of the same element that have different masses.

Look at the diagrams in Fig. 1 below to see the isotopes of hydrogen. The two heavier isotopes of hydrogen are often known as deuterium () and tritium ().

Figure 1.   The isotopes of hydrogen.
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Complete the table below to show the number of protons, neutrons and electrons in each
isotope
Isotopes are atoms of the same element with different numbers of neutrons. They have the same atomic number but different mass numbers.
isotope
of hydrogen:

  • Isotope of hydrogen Number of protons Number of electrons Number of neutrons
    (hydrogen)
    (deuterium)
    (tritium)

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Which statement is true about the isotopes of hydrogen?
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Different isotopes of an element have the same atomic number, but different mass numbers. That's because each isotope of an element contains different numbers of neutrons.

The chemical properties of an element are determined by its
electronic structure
The electronic structure is a description of the arrangement of the electrons in an atom, starting from the lowest energy level (shell). For example, the electronic structure of sodium is 2, 8, 1. (Also known as an electronic configuration.)
electronic structure
. Because isotopes have the same number, and therefore arrangement, of electrons, isotopes of an element undergo the same chemical reactions. The only difference is their mass.

Isotopes are sometimes shown by the name of the element, followed by the
mass number
The mass number is the number of protons plus neutrons in the nucleus of an atom.
mass number
. Thus the three isotopes of hydrogen would be:

Hydrogen-1
Hydrogen-2
Hydrogen-3

Here are two isotopes of carbon: and . How do these two isotopes of carbon differ?
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Which of these statements is true about the reactions of carbon-12 and carbon-14 when heated in pure oxygen?
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In the reaction in the previous question, which statement is true?
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This is a diagram of two carbon isotopes:

Figure 2.   Two isotopes of carbon.
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Calculating relative atomic masses
The mass of atoms can be compared on a relative scale. It is more convenient than quoting the tiny masses of atoms in standard units of mass, like grams or kilograms.

On the relative atomic mass scale, a carbon-12 atom has a mass of exactly 12. For our purposes we can take the mass number as the
relative atomic mass
The relative atomic mass, Ar, is the mass of one mole of atoms of an element (taking into account the proportions of each isotope in a naturally occurring sample of the element).
relative atomic mass
of a particular atom. Therefore, the relative atomic mass of hydrogen-1 is 1, making it the lightest of all atoms.

However, if you look up the mass of elements in a table of data, you will find that chlorine has a relative atomic mass of 35.5. At first sight this seems a bit strange. How can you have an atomic mass that is a fraction? After all, you can't get half a proton or half a neutron in an atom! But the answer lies in the existence of isotopes.

Chlorine exists as two isotopes: chlorine-35 and chlorine-37. When we calculate the relative atomic mass of an element we have to take into account the proportions of each isotope present. In a naturally occurring sample of chlorine, we find that 75 per cent is chlorine-35 atoms and the other 25 per cent is chlorine-37 atoms.

Imagine you have 100 atoms of chlorine: 75 will have a relative atomic mass of 35 and 25 will have a relative atomic mass of 37.
The total mass of the 100 atoms will be:

Total mass  =  (75 × 35)  +  (25 × 37)  =  3550
The mean (average) value of your 100 atom sample will be:

Mean mass  =    =  35.5
 
     So the relative atomic mass of chlorine is 35.5.



Use these three steps to answer this question:

What is the relative atomic mass of thallium which is made up from 30 per cent thallium-203 and 70 per cent thallium-205?
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Summary


Isotopes of an element have the same number of protons and electrons, but different numbers of neutrons.

Different isotopes of an element have different mass numbers, but react chemically in exactly the same way.

The relative atomic mass of an element is the average mass of the isotopes in a naturally occurring sample of the element, taking into account the proportion of each isotope present.

Exercises
1. Isotopes are atoms of the same element with different numbers of …
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Figure 3.   Two isotopes of oxygen.
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2. What is the atomic number of isotope A and isotope B in Fig.3?
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3. Isotope B compared to isotope A has …
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4. Which isotope in Fig.3 has a mass number of 18?
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5. Which of the two isotopes in Fig.3 is the more reactive?
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6. When isotope A reacts with hot sulfur, it makes sulfur dioxide, SO2. What will be formed when isotope B reacts with hot sulfur?
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7. The vast majority of naturally occurring oxygen is in the form of isotope A. Which statement about the relative atomic mass of oxygen is true?
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8. The atomic number of uranium is 92. How many protons, neutrons, and electrons does an atom of uranium-235 contain?
  • protons    electrons    neutrons
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9. How many protons, neutrons, and electrons does an atom of uranium-238 contain?
  • protons    electrons    neutrons
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10. The isotopes of uranium must be separated before they can be used in a nuclear reactor. The method of separation relies on differing rates of progress as a mixture is passed through a long tube. The heavier the atom, the longer it takes to travel down the tube. Which isotope of uranium would get through first?
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