Molécules et composés

Elements are pure substances that cannot be broken down into simpler substances and consist of only one type of atom. Compounds are pure substances made of two or more elements combined in fixed ratios. While elements exist as single atoms or molecular units, compounds form through chemical bonding between different elements.

Atomic elements exist as single atoms, such as helium, neon, and krypton. Molecular elements contain two or more identical atoms bonded together, forming molecules as their base units, like hydrogen or sulfur. Both are classified as elements but differ in their basic structural units.

Covalent bonds form when atoms of two or more nonmetallic elements share valence electrons. Water exemplifies this: one oxygen atom and two hydrogen atoms bond covalently to create a molecular compound. The shared electrons hold the atoms together, forming discrete neutral molecules as the basic units.

Ionic compounds form when metallic elements lose valence electrons to become positively charged cations, while nonmetallic elements gain electrons to become negatively charged anions. These oppositely charged ions attract through electrostatic forces, creating ionic bonds. Table salt exemplifies this: sodium cations and chloride anions combine in a one-to-one ratio.

The four elements common to all living organisms are oxygen, carbon, hydrogen, and nitrogen. While oxygen and nitrogen are abundant in the atmosphere, carbon and hydrogen are relatively rare in Earth's crust overall. Despite these differences in abundance, all elements obey the same chemical and physical laws in both living and nonliving systems.

A pure substance consists of only one type of matter and can be either an element or a compound. Elements contain only one type of atom, while compounds contain two or more types of atoms held together by chemical bonds. Pure substances have consistent chemical and physical properties throughout.

There are 118 known elements, but only 98 occur naturally on Earth. The remaining elements are unstable and require scientists to synthesize them in laboratories. These synthetic elements cannot exist in nature because they decay too rapidly, making laboratory creation necessary for their study.