Isotopes (Cambridge (CIE) O Level Chemistry): Revision Note

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 ${}_6{}^{14}\mathrm{C}$

The Atomic Structure and Symbols of the Three Isotopes of Hydrogen

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

Why Isotopes Share Properties

• Isotopes of the same element display the same chemical characteristics

• This is because they have the same number of electrons in their outer shells and, therefore, the same electronic configuration and this is what determines an atom's chemistry

• The difference between isotopes is the number of 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

• Water made from deuterium oxide is known as 'heavy' water. It has a relative formula mass of 20 compared to 18 for ordinary water, so it is about 11% heavier, but it would look, taste and feel just like normal water

  • However, it wouldn't be a good idea to drink it because it is toxic as it interferes with biochemical reactions in your cells!

Calculating Relative Atomic Mass

Relative Atomic Mass

• The symbol for the relative atomic mass is A_r

• The relative atomic mass for each element can be found in the Periodic Table along with the atomic number

• The atomic number is shown above the atomic symbol and the relative atomic mass is shown below the atomic symbol

• Atoms are too small to accurately weigh but scientists needed a way to compare the masses of atoms 

• The carbon-12 is used as the standard atom and has a fixed mass of 12 units 

• The masses of all other atoms are compared against this atom

• Relative atomic mass (A_r) can therefore be defined as:

  • the average mass of the isotopes of an element compared to 1/12th of the mass of an atom of ¹²C 

• The relative atomic mass of carbon is 12

  • The relative atomic mass of magnesium is 24 which means that magnesium is twice as heavy as carbon 

  • The relative atomic mass of hydrogen is 1 which means it has one-twelfth the mass of one carbon-12 atom 

• The relative atomic mass of an element can be calculated from the mass number and relative abundances of all the isotopes of a particular element using the following equation:

$A_r=\frac{(\%\mathrm{of}\mathrm{isotope}1\mathrm{x}\mathrm{mass}\mathrm{number}\mathrm{of}\mathrm{isotope}1)+(\%\mathrm{of}\mathrm{isotope}2\mathrm{x}\mathrm{mass}\mathrm{number}\mathrm{of}\mathrm{isotope}2)}{100}$

• The top line of the equation can be extended to include the number of different isotopes of a particular element present.

Example

The table shows information about the isotopes in a sample of rubidium

$A_r=\frac{\left(72\mathrm{x}85\right)+\left(28\mathrm{x}87\right)}{100}=85.6$

Are 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 relative atomic mass is 7, which is the same as the mass number shown on this isotope of lithium