The electronic arrangements of the first 20 elements

The electronic arrangements of the first 20 elements   After this the pattern alters as you enter the transition series in the Periodic Table. Two important generalisations If you look at the patterns in this table:  The number of electrons in the outer level is the same as the group number. (Except with helium which has only 2 electrons. The noble gases are also usually called group 0 - not group 8.) This pattern extends throughout the Periodic Table for the main groups (i.e. not including the transition elements). So if you know that barium is in group 2, it has 2 electrons in its outer level; iodine (group 7) has 7 electrons in its outer level; lead (group 4) has 4 electrons in its outer level. Noble gases have full outer levels. This generalisation will need modifying for A'level purposes. Dots-and-crosses diagrams In any introductory chemistry course you will have come across the electronic structures of hydrogen and carbon, ,