Arrangement of elements
Periods, groups, and the link between position and electronic configuration.
Review these first
Learning objectives
What you need to be able to do
Teacher-mapped phrasing — check against the official Cambridge syllabus for exact wording.
- 8.1.1Describe the Periodic Table as an arrangement of elements in order of proton number.
- 8.1.2Relate group number to outer-shell electrons and period number to the number of occupied shells.
5 minute read
Arrangement of elements in the Periodic Table
The Periodic Table arranges every known element in order of increasing proton number, from hydrogen (1) onward.
Groups and periods
Elements are arranged into vertical groups (numbered I to VIII/0) and horizontal periods. This layout is not arbitrary — it directly reflects electronic configuration:
- The group number equals the number of electrons in an atom's outer shell (for Groups I–VII).
- The period number equals the number of electron shells that are occupied.
This is why elements in the same group share similar chemical properties: they have the same number of outer-shell electrons, and it is largely the outer shell that determines how an atom reacts.
Why the pattern repeats
As proton number increases, electron shells fill up in a repeating pattern (2, 8, 8, 2 for the first 20 elements). Once a shell is completely full, the next electron starts a new shell, beginning a new period — which is exactly why the table's rows are the lengths they are, and why chemical properties recur periodically as you move across it.
Think of it like this
The Periodic Table is like a filing cabinet where the drawer (period) tells you how many shelves of storage (electron shells) an item uses, and the position along a shelf (group) tells you what is on the very top layer (outer-shell electrons) — items in the same position on different shelves behave alike because their top layer is arranged the same way.
Worked examples
Method, step by step
An atom has the electronic configuration 2,8,2. State its group and period.
- 1Outer shell has 2 electrons, so Group II
- 23 shells are occupied, so Period 3
Group II, Period 3 (this is magnesium)
Common misconceptions
- Thinking elements are arranged by relative atomic mass. They are arranged by increasing proton number — mass generally increases alongside this, but proton number is the defining order.
- Assuming group number always equals outer-shell electrons for every element. This rule applies cleanly to Groups I–VII (the main-group elements); transition elements do not follow it simply.
- Confusing group and period — remembering "group down, period across" is a fast way to keep the two straight.
In the exam
- When asked to justify an element's group or period from its electronic configuration, state the rule explicitly: "N outer electrons means Group N" and "M occupied shells means Period M".
- Elements in the same group are asked about constantly for their "similar properties" — always trace this back to their shared number of outer-shell electrons.