Atom | Definition, Structure, History, Examples, Diagram, & Facts

Ask the Chatbot Games & Quizzes History & Society Science & Tech Biographies Animals & Nature Geography & Travel Arts & Culture ProCon Money Videos atom

• Introduction & Top Questions
• Atomic model
  • Basic properties
    • Atomic number
    • Atomic mass and isotopes
  • The electron
    • Charge, mass, and spin
    • Orbits and energy levels
    • Electron shells
    • Atomic bonds
    • Conductors and insulators
    • Magnetic properties
  • The nucleus
    • Nuclear forces
    • Nuclear shell model
    • Radioactive decay
    • Nuclear energy
• Development of atomic theory
  • The atomic philosophy of the early Greeks
  • The emergence of experimental science
  • The beginnings of modern atomic theory
    • Experimental foundation of atomic chemistry
    • Atomic weights and the periodic table
    • Kinetic theory of gases
  • Studies of the properties of atoms
    • Size of atoms
    • Electric properties of atoms
    • Light and spectral lines
    • Discovery of electrons
    • Identification of positive ions
    • Discovery of radioactivity
  • Models of atomic structure
    • Rutherford’s nuclear model
    • Moseley’s X-ray studies
    • Bohr’s shell model
    • The laws of quantum mechanics
    • Schrödinger’s wave equation
    • Antiparticles and the electron’s spin
  • Advances in nuclear and subatomic physics
    • Structure of the nucleus
    • Quantum field theory and the standard model

References & Edit History Related Topics Images & Videos For Students atom summary Quizzes Facts You Should Know: The Periodic Table Quiz Related Questions

• How is the atomic number of an atom defined?
• What are the three types of atoms?
• When are isotopes stable?

Science Physics Matter & Energy atom matter Quick Summary Ask the Chatbot Print print Print Please select which sections you would like to print:

• Table Of Contents

Cite verifiedCite While every effort has been made to follow citation style rules, there may be some discrepancies. Please refer to the appropriate style manual or other sources if you have any questions. Select Citation Style MLA APA Chicago Manual of Style Copy Citation Share Share Share to social media Facebook X URL https://www.britannica.com/science/atom Feedback External Websites Feedback Corrections? Updates? Omissions? Let us know if you have suggestions to improve this article (requires login). Feedback Type Select a type (Required) Factual Correction Spelling/Grammar Correction Link Correction Additional Information Other Your Feedback Submit Feedback Thank you for your feedback

Our editors will review what you’ve submitted and determine whether to revise the article.

External Websites

• Institute of Physics - The atom
• University of Central Florida Pressbooks - Chemistry Fundamentals - Quantum Mechanics and The Atom
• Energy Education - Atom
• Open Oregon Educational Resources - Elements and Atoms: The Building Blocks of Matter
• Mustansiriyah University - Fundamental particles of an atom (PDF)
• Space.com - Atoms: What are they and how do they build the elements?
• Annenberg Learner - The Behavior of Atoms: Phases of Matter and the Properties of Gases
• UEN Digital Press with Pressbooks - The Structure of the Atom
• United States Nuclear Regulatory Commission - What is an atom ?
• Projects at Harvard - Atomic Structure & Chemical Bonding
• Chemistry LibreTexts - Atoms
• Live Science - What Is an Atom? Facts About the Building Blocks of the Universe
• Khan Academy - Introduction to the atom

Britannica Websites Articles from Britannica Encyclopedias for elementary and high school students.

• atom - Children's Encyclopedia (Ages 8-11)
• atom - Student Encyclopedia (Ages 11 and up)

Quick Summary Ask the Chatbot a Question Written by James Trefil Clarence J. Robinson Professor of Physics, George Mason University, Fairfax, Virginia. Author of Science Matters: Achieving Scientific Literacy; Other Worlds: The Solar System and Beyond; and Encyclopedia... James TrefilAll Fact-checked by Britannica Editors Encyclopaedia Britannica's editors oversee subject areas in which they have extensive knowledge, whether from years of experience gained by working on that content or via study for an advanced degree.... Britannica Editors Last updated Dec. 25, 2025 •History Table of Contents Table of Contents Quick Summary Ask the Chatbot a Question Top Questions

What is an atom?

An atom is the basic building block of chemistry. It is the smallest unit into which matter can be divided without the release of electrically charged particles. It also is the smallest unit of matter that has the characteristic properties of a chemical element.

Are all atoms the same size?

All atoms are roughly the same size, whether they have 3 or 90 electrons. Approximately 50 million atoms of solid matter lined up in a row would measure 1 cm (0.4 inch). A convenient unit of length for measuring atomic sizes is the angstrom, defined as 10−10 meters.

What does the mass of an atom consist of?

The mass of an atom consists of the mass of the nucleus plus that of the electrons. That means the atomic mass unit is not exactly the same as the mass of the proton or neutron.

How is the atomic number of an atom defined?

The single most important characteristic of an atom is its atomic number (usually denoted by the letter Z), which is defined as the number of units of positive charge (protons) in the nucleus. For example, if an atom has a Z of 6, it is carbon, while a Z of 92 corresponds to uranium.

Are humans made of atoms?

Yes, humans are made of atoms. Most of the human body is made up of atoms of four elements: oxygen, carbon, hydrogen, and nitrogen. These four elements are about 96 percent of a human’s weight.

What are the three types of atoms?

There are not three types of atoms. Each chemical element in the periodic table has its own distinct kind of atom, and thus there are 118 types of atoms. However, atoms are made up of three types of subatomic particles: the heavy protons and neutrons that make up the nucleus (the central part of the atom), and the much lighter electrons that circle the nucleus in orbital paths called shells.

News •

Physicists made atoms behave like a quantum circuit • Dec. 22, 2025, 10:15 PM ET (ScienceDaily) ...(Show more) Physicists found a way to see heat in empty space • Dec. 16, 2025, 10:40 PM ET (ScienceDaily) Show less

atom, the basic building block of all matter and chemistry. Atoms can combine with other atoms to form molecules but cannot be divided into smaller parts by ordinary chemical processes.

Most of the atom is empty space. The rest consists of three basic types of subatomic particles: protons, neutrons, and electrons. The protons and neutrons form the atom’s central nucleus. (The ordinary hydrogen atom is an exception; it contains one proton but no neutrons.) As their names suggest, protons have a positive electrical charge, while neutrons are electrically neutral—they carry no charge; overall, then, the nucleus has a positive charge. Circling the nucleus is a cloud of electrons, which are negatively charged. Like opposite ends of a magnet that attract one another, the negative electrons are attracted to a positive force, which binds them to the nucleus. The nucleus is small and dense compared with the electrons, which are the lightest charged particles in nature. The electrons circle the nucleus in orbital paths called shells, each of which holds only a certain number of electrons.

An ordinary, neutral atom has an equal number of protons (in the nucleus) and electrons (surrounding the nucleus). Thus the positive and negative charges are balanced. Some atoms, however, lose or gain electrons in chemical reactions or in collisions with other particles. Ordinary atoms that either gain or lose electrons are called ions. If a neutral atom loses an electron, it becomes a positive ion. If it gains an electron, it becomes a negative ion. These basic subatomic particles—protons, neutrons, and electrons—are themselves made up of smaller substances, such as quarks and leptons.

More than 90 types of atoms exist in nature, and each kind of atom forms a different chemical element. Chemical elements are made up of only one type of atom—gold contains only gold atoms, and neon contains only neon atoms--and they are ranked in order of their atomic number (the total number of protons in its nucleus) in a chart called the periodic table. Accordingly, because an atom of iron has 26 protons in its nucleus, its atomic number is 26 and its ranking on the periodic table of chemical elements is 26. Because an ordinary atom has the same number of electrons as protons, an element’s atomic number also tells how many electrons its atoms have, and it is the number and arrangement of the electrons in their orbiting shells that determines how one atom interacts with another. The key shell is the outermost one, called the valence shell. If this outermost shell is complete, or filled with the maximum number of electrons for that shell, the atom is stable, with little or no tendency to interact with other atoms. But atoms with incomplete outer shells seek to fill or to empty such shells by gaining or losing electrons or by sharing electrons with other atoms. This is the basis of an atom’s chemical activity. Atoms that have the same number of electrons in the outer shell have similar chemical properties.

This article opens with a broad overview of the fundamental properties of the atom and its constituent particles and forces. Following this overview is a historical survey of the most influential concepts about the atom that have been formulated through the centuries.

Britannica Quiz Facts You Should Know: The Periodic Table Quiz

Atomic model

Most matter consists of an agglomeration of molecules, which can be separated relatively easily. Molecules, in turn, are composed of atoms joined by chemical bonds that are more difficult to break. Each individual atom consists of smaller particles—namely, electrons and nuclei. These particles are electrically charged, and the electric forces on the charge are responsible for holding the atom together. Attempts to separate these smaller constituent particles require ever-increasing amounts of energy and result in the creation of new subatomic particles, many of which are charged.

As noted in the introduction to this article, an atom consists largely of empty space. The nucleus is the positively charged center of an atom and contains most of its mass. It is composed of protons, which have a positive charge, and neutrons, which have no charge. Protons, neutrons, and the electrons surrounding them are long-lived particles present in all ordinary, naturally occurring atoms. Other subatomic particles may be found in association with these three types of particles. They can be created only with the addition of enormous amounts of energy, however, and are very short-lived.

Access for the whole family! Bundle Britannica Premium and Kids for the ultimate resource destination. Subscribe

All atoms are roughly the same size, whether they have 3 or 90 electrons. Approximately 50 million atoms of solid matter lined up in a row would measure 1 cm (0.4 inch). A convenient unit of length for measuring atomic sizes is the angstrom (Å), defined as 10−10 meter. The radius of an atom measures 1–2 Å. Compared with the overall size of the atom, the nucleus is even more minute. It is in the same proportion to the atom as a marble is to a football field. In volume the nucleus takes up only 10−14 meters of the space in the atom—i.e., 1 part in 100,000. A convenient unit of length for measuring nuclear sizes is the femtometer (fm), which equals 10−15 meter. The diameter of a nucleus depends on the number of particles it contains and ranges from about 4 fm for a light nucleus such as carbon to 15 fm for a heavy nucleus such as lead. In spite of the small size of the nucleus, virtually all the mass of the atom is concentrated there. The protons are massive, positively charged particles, whereas the neutrons have no charge and are slightly more massive than the protons. The fact that nuclei can have anywhere from 1 to nearly 300 protons and neutrons accounts for their wide variation in mass. The lightest nucleus, that of hydrogen, is 1,836 times more massive than an electron, while heavy nuclei are nearly 500,000 times more massive.

Key People: Ernest Rutherford Niels Bohr Lev Davidovich Landau Steven Chu William D. Phillips (Show more) Related Topics: subatomic particle atomic particle line spectrum particle radiation allotrope (Show more) See all related content

Basic properties

Atomic number

The single most important characteristic of an atom is its atomic number (usually denoted by the letter Z), which is defined as the number of units of positive charge (protons) in the nucleus. For example, if an atom has a Z of 6, it is carbon, while a Z of 92 corresponds to uranium. A neutral atom has an equal number of protons and electrons so that the positive and negative charges exactly balance. Since it is the electrons that determine how one atom interacts with another, in the end it is the number of protons in the nucleus that determines the chemical properties of an atom.