This page explains that elements are pure substances composed of one type of atom, crucial for life, with only around 25 of Earth's 90 elements being essential. It details atomic structure, focusing o...This page explains that elements are pure substances composed of one type of atom, crucial for life, with only around 25 of Earth's 90 elements being essential. It details atomic structure, focusing on the nucleus and outer electrons, which dictate chemical behavior, valence, and electronegativity. Elements with similar outer electron configurations share chemical properties and react to achieve stability.