Description: Atoms are the smallest building blocks of matter that constitute any given chemical element. For example, helium gas consists of a collection of many helium atoms, each such helium atom indistinguishable from the next; carbon in the form of, say, diamond, is a collection of carbon atoms.

The word "atom" goes back to Democritus, an Ancient Greek philosopher, who postulated that atoms are the indivisible fundamental components for all of matter. In contrast, atoms according to the modern definition are made up of electrons, protons, and neutrons. Protons and neutrons form the atomic nucleus. All atoms of a given chemical element have the same number of protons, uniquely characteristic for that element. The atomic nucleus is surrounded by electrons. As the term is commonly used, atoms are electrically neutral, with as many protons (each with unit positive electric charge) as electrons (each with unit negative charge). When electrons are taken away from, or added to, this neutral configuration, an atom becomes an ion.

The electron shell determines the chemical properties of an atom. The differences and similarities between the different kinds of atoms are organized and classified in the periodic table of chemical elements. The electron shell also determines how an atom interacts with radiation. The resulting features in the spectrum of light received from astronomical objects allow astronomers to identify the different kinds of atoms in space.

Related Terms:

• Atomic Structure
• Ionization
• Electron
• Spectral Line
• Absorption Line
• Emission Line

Term and definition status: This term and its definition have been approved by a research astronomer and a teacher

The OAE Multilingual Glossary is a project of the IAU Office of Astronomy for Education (OAE) in collaboration with the IAU Office of Astronomy Outreach (OAO). The terms and definitions were chosen, written and reviewed by a collective effort from the OAE, the OAE Centers and Nodes, the OAE National Astronomy Education Coordinators (NAECs) and other volunteers. You can find a full list of credits here. All glossary terms and their definitions are released under a Creative Commons CC BY-4.0 license and should be credited to "IAU OAE".