Elements & Isotopes


An element is a pure substance that cannot be decomposed into anything simpler by a chemical reaction.

All known elements are arranged in order of increasing proton number in the Periodic Table. The reason why they can do that is because different elements have proton numbers. This means that any element can be identified by its proton number alone. (Proton number = fingerprint of an element)

Elements in the Periodic Table is represented by a symbol consisting of one or two letters. The first letter is always capitalised.

Some common elements and their symbol:

Element Symbol
Hydrogen H
Carbon C
Nitrogen N
Oxygen O
Aluminium Al


Elements are divided into two groups – metal and non-metal. Metals are found on the left side of the Periodic Table, while the non-metals are on the right side. (More about this in the section dedicated to Periodic Table)



Isotopes are different atoms of an element which have the same proton number but different nucleon numbers.

  • They can be considered as the very close “siblings” of an element.
  • The chemical properties of an element and its isotopes are the same and cannot be distinguish from one another. This is due to the element and its isotopes having the same number of electrons. (Remember that number of electrons = number of protons in elements as elements are neutral)
  • However, the physical properties of isotopes can be different. For instance, they might have slightly different boiling points.


Common isotopes are given special names. Example: Isotopes of hydrogen: (Hydrogen has 3 naturally occurring isotopes)

  • $^{1}_{1}\text{H}$  Protium or Hydrogen-1
  • $^{2}_{1}\text{H}$  Deuterium or Hydrogen-2
  • $^{3}_{1}\text{H}$  Tritium or Hydrogen-3

Notice that the proton number is the same. Only the number of neutrons are different among the isotopes.


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