Isotopes (Cambridge (CIE) IGCSE Chemistry)

Defining isotopes

• Isotopes are different atoms of the same element that contain the same number of protons but a different number of neutrons

• The symbol for an isotope is the chemical symbol (or word) followed by a dash and then the mass number

• So C-14 ( or carbon-14) is the isotope of carbon which contains 6 protons, 6 electrons and 14 - 6 = 8 neutrons

  • It can also be written as ¹⁴C or

Table to show the structures of isotopes of hydrogen

Isotope	Atomic Structure	Symbol
Hydrogen - 1
Hydrogen - 2
Hydrogen - 3

Why isotopes share properties

Extended tier only 

• Isotopes display the same chemical characteristics

• This is because they have the same number of electrons in their outer shells, and this is what determines their chemistry

• The difference between isotopes is the neutrons which are neutral particles within the nucleus and add mass only

• The difference in mass affects the physical properties, such as density, boiling point and melting point

• Isotopes are identical in appearance, so a sample of C-14 would look no different from C-12

Calculating relative atomic mass

Extended tier only 

Relative atomic mass

• Atoms are so tiny that we cannot really compare their masses in conventional units such as kilograms or grams, so a unit called the relative atomic mass (A_r) is used

• The relative atomic mass unit is equal to 1/12th the mass of a carbon-12 atom

• All other elements are measured by comparison to the mass of a carbon-12 atom and since these are ratios, the relative atomic mass has no units

• For example, hydrogen has a relative atomic mass of 1, meaning that 12 atoms of hydrogen would have exactly the same mass as 1 atom of carbon

How do I calculate relative atomic mass?

• The relative atomic mass of each element is calculated from the mass number and relative abundances of all the isotopes of a particular element

• The equation below is used where the top line of the equation can be extended to include the number of different isotopes of a particular element present

• So, if there were 3 isotopes present then the equation would read:

Is mass number and relative atomic mass the same thing?

• On the Periodic Table provided in your exam you will see that lithium has a relative atomic mass of 7

• Although it seems that this is the same as the mass number, they are not the same thing because the relative atomic mass is a rounded number

• Relative atomic mass takes into account the existence of isotopes when calculating the mass

• Relative atomic mass is an average mass of all the isotopes of that element

• For simplicity relative atomic masses are often shown to the nearest whole number

The relative atomic mass of lithium to two decimal places is 6.94 when rounded to the nearest whole number, the RAM is 7, which is the same as the mass number shown on this isotope of lithium