Reading position from the shells

A period is a horizontal row and a group is a vertical column. In an electron arrangement such as 2.8.7, each number gives the electrons in one occupied shell.

For neutral atoms of the first 20 elements:

• The number of occupied shells gives the period: 2.8.7 has three occupied shells, so it is in Period 3.
• In the course's main-group numbering, Groups 1–7 match the number of outer-shell electrons: 2.8.7 ends in 7, so it is in Group 7.
• Group 0 contains the noble gases, which have full outer shells. Helium's single shell is full with 2 electrons; neon is 2.8 and argon is 2.8.8. Group 0 does not mean zero outer electrons.

Some periodic tables use Groups 13–18 for the columns called 3–7 and 0 here. In these lessons, use the stated 1–7 and 0 convention.

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The diagram compares Periods 2 and 3 with the central space omitted. Lithium (2.1) and sodium (2.8.1) each have one outer electron and belong to Group 1, but sodium has one more occupied shell and belongs to the next period. Their common outer-electron pattern helps explain their similar chemical behaviour.

Do not add all the numbers to find the period. The sum gives the electron total and hence the atomic number of a neutral atom.

Working in both directions

From arrangement to position

Calcium has arrangement 2.8.8.2. There are four occupied shells, so it is in Period 4. Its two outer electrons put it in Group 2. Use both pieces of information: the final number alone does not tell you the period.

From position to arrangement

An element in Period 3 and Group 5 has three occupied shells and five electrons in its outer shell. The inner shells contain 2 and 8 electrons, so the arrangement is 2.8.5. Its total is 15, the atomic number of the neutral atom.

For a Group 0 element, use a full outer shell. Period 2 and Group 0 therefore gives 2.8, not 2.0. Always use the arrangement of the neutral atom when locating the element; an ion may have lost or gained electrons.