Here's why:
* Groups on the periodic table are vertical columns. Elements in the same group share similar chemical properties because they have the same number of valence electrons.
* Valence electrons are the electrons in the outermost energy level of an atom, and they are the ones involved in chemical bonding.
Example:
* Oxygen is in Group 16 (also known as VIA). This means it has 6 valence electrons.
* Sodium is in Group 1 (also known as IA). This means it has 1 valence electron.
Important Note: This rule applies to the main group elements (Groups 1, 2, and 13-18). Transition metals (Groups 3-12) have more complex electron configurations and their valence electron count can be more variable.
