The image displays a periodic table illustrating how to identify Group Numbers and Period Numbers.

The image displays a periodic table illustrating how to identify Group Numbers and Period Numbers.

• Period Numbers: These are the horizontal rows on the periodic table, numbered from 1 to 7.
  • Elements in the same period have the same number of electron shells. For example, all elements in Period 3 have three electron shells.
• Group Numbers: These are the vertical columns on the periodic table, numbered from 1 to 18.
  • Elements in the same main group (Groups 1, 2, and 13-18, also known as representative elements) generally have the same number of valence electrons and thus similar chemical properties. For example, all elements in Group 1 (excluding hydrogen) are alkali metals and have one valence electron.