| 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. |