Period number

The period number is the number of the horizontal row an element sits in; it equals the number of occupied electron shells in an atom of that element. Malay: nombor kala · Chinese: 周期数.

The period number is the partner of the group number, and together they read an atom’s electron arrangement straight off the table. Where the group number counts the valence electrons, the period number counts the shells those electrons are spread across. So if you are told an element is in Period 3, you know at once that its atoms use three occupied shells, and the last of those three is the valence shell that decides its chemistry.

Example. Sodium has the arrangement 2.8.1: three numbers, so three occupied shells, so Period 3. Lithium is 2.1: two numbers, so two occupied shells, so Period 2. Potassium is 2.8.8.1: four numbers, so Period 4. In every case you simply count how many shells hold electrons, and that count is the period number, you can even work it backwards, using the period to check your electron arrangement.

Confusion to avoid. The period number is not the number of valence electrons, that is the group number. Period 3 does not mean three valence electrons; it means three shells. Students who mix the two often place elements in the wrong row or the wrong column, so hold the rule firmly: shells give the period, outer-shell electrons give the group.

In the Periodic Table of Elements chapter the period number and group number together are the two coordinates that locate any of the first twenty elements exactly, turning a memorised arrangement into a position you can reason about. Because the syllabus concentrates on those first twenty elements, this counting habit is enough to place almost every element you will meet in the paper.