| 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.
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