Working out the electron arrangement

Electrons are arranged in shells or energy levels around the nucleus of an atom. The number of electrons in an atom is equal to the number of protons. The chemical properties of an element are determined by the arrangement of electrons in its atom.

Filling the shells

The innermost shell (lowest energy level), which can contain a maximum of two electrons, is filled first. On filling the first shell, the second shell or energy level is filled. This can hold a maximum of eight electrons. The next shell, i.e. the third shell fills. This can also hold a maximum of 8 electrons. Then fourth shell begins to fill after that.



Working out the electronic arrangement from the Periodic Table

  1. Locate the element in the Periodic Table.
  2. Determine the period it belongs to (periods are the rows across the Periodic Table). The period represents the number of shells or energy levels, draw that number of circles around the nucleus i.e. if the element is located in period two (row number 2) then draw two circles around then nucleus.
  3. Find out the Group number from the Periodic Table (the group is the vertical column in the Periodic Table). The group number represents the number of electron in the outermost shell, draw that number of electrons in the outermost circle. (Remember: With the exception of Helium that has only 2 electrons, all other elements of Group 0 have 8 electrons in their outermost shell.)
  4. Fill the other unfilled circles with electrons using the maximum number of electrons it can hold, i.e. 2 in the first and 8 in second and third shells.
  5. Count the number of your electrons and verify that it is equal to the atomic number of that element.