Transactinides

While groups are characterised by the number of electrons present in the outer shell, periods are characterized by the number of energy levels (shells) of electrons surrounding the nucleus . Elements in Period 1 has only one shell. As you probably recall, the elements in the first period have a 2 electrons maximum ( hydrogen has 1 electron and helium has 2 electrons. As we move to the first group of the second period, we find that lithium , which has the two electrons in the first shell and one in the second. Neon is in Group 18 of Period 2 and therefore has the two electrons in the first shell and eight electrons in the second shell. Sodium starts Period 3 with 11 electrons , two in the first shell, eight in the second shell and one in the third shell. In other words, the element in Group 1 always has one more electron (in a new shell) than the Group 18 element in the previous period.

See Also

Actinides and Rare Earth Elements .