4.2 - The Arrangement in the Periodic Table of Elements

Modern Periodic Table

• Elements are arranged systematically in ascending proton number from left to right and from top to bottom.
• A vertical column is a group; the table has 18 groups.
• A horizontal row is a period; the table has seven periods.

Electron Arrangement, Group and Period

• The number of valence electrons determines the group for main-group elements:

  • 1 or 2 valence electrons: group number equals the number of valence electrons;
  • 3–8 valence electrons: group number equals the number of valence electrons $+10$.

• The number of electron-filled shells determines the period.

• Helium is an exception to the simple group rule: it has two valence electrons but is in Group 18 because its first shell is full.

Element (proton number)	Electron arrangement	Group, period
Li (3)	$2.1$	Group 1, Period 2
Ca (20)	$\mathrm{2.8.8.2}$	Group 2, Period 4
Al (13)	$\mathrm{2.8.3}$	Group 13, Period 3
Si (14)	$\mathrm{2.8.4}$	Group 14, Period 3
N (7)	$2.5$	Group 15, Period 2
O (8)	$2.6$	Group 16, Period 2
Br (35)	$\mathrm{2.8.18.7}$	Group 17, Period 4
Kr (36)	$\mathrm{2.8.18.8}$	Group 18, Period 4