<em><u>The atom of a chemical element is made up of sub-atomic particles which are the neutrons, protons and electrons.</u></em> The neutrons and protons occupies the nucleus of an atom while the electrons occupy the energy levels.
<em><u>The first energy level (n=1) of an atom occupies a maximum of two electrons because it has only an S-orbital which can only have two electrons.</u></em>
<u>The second energy level (n=2) can have a maximum number of 8 electrons, since it contains, one S- orbital which can hold two electrons, and a p-orbital which can accommodate six electrons.</u>
<em><u>The third energy level (n=3) can have a maximum number of 18 electrons,</u></em> because it has <u><em>the S-orbitals which holds 2 electrons, the p-orbital which holds 6 electrons and the d-orbital which holds 10 electrons. </em></u>