Electron Configuration

The chemistry of each element is determined by its electron configuration, in particular, the number of electrons in the outermost (highest energy) shell. As you go down a group, there is an extra shell, but the same number of electrons in the outermost shell.The periodic table can be divided into blocks. The s-block, p-block, d-block and f-block correspond to the highest energy shell of the elements within the block.The old numbering system numbers group 1 - 7 and then 0 which matches the number of electrons in the highest energy shell.The new system numbers the groups from 1 to 18.