Here's why:
* Valence electrons are the electrons in the outermost shell of an atom, and they are the ones involved in chemical bonding.
* Groups in the periodic table are organized based on the number of valence electrons an element has. For example, all elements in Group 1 (alkali metals) have 1 valence electron, while all elements in Group 17 (halogens) have 7 valence electrons.
Example:
* Lithium (Li), Sodium (Na), Potassium (K) are all in Group 1 and have 1 valence electron each.
* Oxygen (O), Sulfur (S), Selenium (Se) are all in Group 16 and have 6 valence electrons each.
