The periodic table orders elements by increasing atomic number, the proton count. Neutral atoms have the same number of electrons as protons. For the first 20 elements, a useful simple shell arrangement fills the first shell to two electrons, then represents the next shells with up to eight before the next begins. Sodium, atomic number 11, is represented as 2,8,1.
A period is a horizontal row and corresponds to occupied electron shells in this simple model. Elements in a main-group column share related outer-electron patterns and often similar chemical behaviour. Group 1 has one outer electron; group 17 has seven. Group 18 has a full outer shell, including helium’s full two-electron first shell. This simplified rule is not a full account of transition-metal electron arrangements.
Example 1
Give the shell arrangement and period for neutral magnesium, atomic number 12.
Twelve electrons arrange as 2,8,2 in the simple model. Three occupied shells place magnesium in period 3.
Example 2
A period−3 atom has seven outer electrons. Identify its group and simple arrangement.
Seven outer electrons correspond to group 17. With three shells its arrangement is 2,8,7, identifying chlorine.
Watch out: Periods are horizontal rows; groups are vertical columns.