How the table is built
The Periodic Table lists all the known elements in order of increasing proton number (atomic number). This arrangement places elements with similar chemical properties into the same vertical columns.
Key idea
Groups are the vertical columns; elements in the same group have the same number of outer-shell electrons and so show similar chemical properties. Periods are the horizontal rows; the period number equals the number of occupied electron shells.
Reading position from structure
- The group number (for main groups) equals the number of electrons in the outer shell.
- The period number equals the number of electron shells that contain electrons.
- Metals are on the left and centre; non-metals are on the upper right; the noble gases form Group VIII (Group 0) on the far right.
Worked example
An atom has the electronic configuration 2,8,2. Where is it in the Periodic Table?
Outer shell has 2 electrons → Group II. There are 3 occupied shells → Period 3. So the element is magnesium (Mg), proton number 12.
Remember
- Order = proton number, NOT relative atomic mass.
- Same group → similar properties; same period → same number of shells.