Pure substances consist of only one type of matter. A pure substance can be an element or a compound. An element consists of only one type of atom, while a compound consists of two or more types of atoms held together by a chemical bond.

Elements

Elements are classified as atomic or molecular based on the nature of their basic units. They are unique forms of matter with specific chemical and physical properties that cannot break down into smaller substances by ordinary chemical reactions. There are 118 elements, but only 98 occur naturally. The remaining elements are unstable and require scientists to synthesize them in laboratories. The four elements common to all living organisms are oxygen (O), carbon (C), hydrogen (H), and nitrogen (N). In the nonliving world, elements are found in different proportions, and some elements common to living organisms are relatively rare on the earth as a whole. For example, the atmosphere is rich in nitrogen and oxygen but contains little carbon and hydrogen, while the earth's crust has little nitrogen and carbon, although it contains oxygen and a small amount of hydrogen. In spite of their differences in abundance, all elements and the chemical reactions between them obey the same chemical and physical laws regardless of whether they are a part of the living or nonliving world.

Compounds
Compounds are pure substances composed of two or more elements in fixed, definite proportions. Compounds are classified as molecular (covalent) or ionic based on the bonds present in them.

Molecular compounds (or covalent compounds) result when two or more different nonmetal atoms share electrons to form covalent bonds. The basic units of molecular compounds are discrete neutral molecules composed of different constituent atoms. For example, methanol contains CH₃OH molecules as base units, consisting of one carbon atom, one oxygen atom, and four hydrogen atoms, all of which are covalently connected.

A compound that contains ions and is held together by ionic bonds is called an ionic compound. When an element composed of atoms that readily lose electrons (a metal) reacts with an element composed of atoms that readily gain electrons (a nonmetal), a transfer of electrons usually occurs, producing ions. The compound formed by this transfer is stabilized by the electrostatic attractions (ionic bonds) between the ions of opposite charge present in the compound. For example, when each sodium atom gives up one electron to form a sodium cation (Na⁺) and each chlorine atom accepts one electron to form a chloride anion (Cl⁻), the resulting compound NaCl is composed of sodium ions and chloride ions in the ratio of one Na⁺ ion for each Cl⁻ ion.

This text is adapted from Openstax, Biology 2e, Section 2.1: Atoms, Isotopes, Ions and Molecules: The Building Blocks and Openstax, Chemistry 2e, Section 2.6: Molecular and Ionic Compounds.

Reference

1. Clark, M. A., Douglas, M., Choi, J. Section 2.1: Atoms, Isotopes, Ions and Molecules: The Building Blocks. In Biology 2e. OpenStax. Houston, TX (2018).
2. Clark, M. A., Douglas, M., Choi, J. Section 2.6: Molecular and Ionic Compounds. In Chemistry 2e. OpenStax. Houston, TX (2018).