Isotopes (SQA National 5 Chemistry): Revision Note

Defining isotopes

• Isotopes are atoms of the same element 

  • They have the same number of protons (same atomic number)

  • They have a different number of neutrons (different mass number)

• Isotopes also have the same number of electrons

  • This means that they have the same chemical properties

• An isotope can be written in two main ways:

  1. In writing:

     • The name or chemical symbol of the atom followed by a dash and then the mass number

     • For example, carbon-14 or C-14

  2. As a nuclide notation

     • The nuclide notation shows the same atomic number, but a different mass number

     • For example:

       • Carbon-14 has the nuclide notation

       • Carbon-12 has the nuclide notation

Isotopes of chlorine

• Chlorine has two common isotopes:

• Both atoms have 17 protons, which is why they are both chlorine

  • They just have a different number of neutrons

Relative atomic mass

• Atoms are incredibly small, so weighing them in grams is not practical

  • Instead, chemists use a relative scale to compare their masses

• Most elements exist as a mixture of isotopes

• The relative atomic mass is the weighted average mass of an element's atoms

  • This takes into account the proportion (or abundance) of each isotope

• This is why the relative atomic masses found on page 7 of the Data Booklet are not all whole numbers

Understanding the average

• The relative atomic mass is always closer to the mass number of the most common (abundant) isotope

• For example, chlorine has two main isotopes:

chlorine-35:

chlorine-37:

• If they were equally abundant, the average mass would be exactly in the middle:

= 36

• However, the relative atomic mass of chlorine listed in the Data Booklet is 35.5

• Looking at this on a number line, we can see that the average mass (35.5) is closer to 35 than 37:

• So, the chlorine-35 isotope is more abundant than chlorine-37