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A way to remember their size is this: if an apple is magnified to the size of the earth, then the atoms in the apple are approximately the size of the original apple. — Richard Feynman, The Feynman Lectures on Physics




atom : the smallest particle of an element that can exist either alone or in combination — Webster   See also OneLook


Roget’s II (, Merriam-Webster Thesaurus, Visuwords


Atom is the smallest constituent unit of ordinary matter that has the properties of a chemical element. Every solid, liquid, gas, and plasma is composed of neutral or ionized atoms. Atoms are extremely small; typical sizes are around 100 picometers (a ten-billionth of a meter, in the short scale).

Every atom is composed of a nucleus and one or more electrons bound to the nucleus. The nucleus is made of one or more protons and typically a similar number of neutrons. Protons and neutrons are called nucleons. More than 99.94% of an atom’s mass is in the nucleus. The protons have a positive electric charge, the electrons have a negative electric charge, and the neutrons have no electric charge. If the number of protons and electrons are equal, that atom is electrically neutral. If an atom has more or fewer electrons than protons, then it has an overall negative or positive charge, respectively, and it is called an ion.

The electrons of an atom are attracted to the protons in an atomic nucleus by this electromagnetic force. The protons and neutrons in the nucleus are attracted to each other by a different force, the nuclear force, which is usually stronger than the electromagnetic force repelling the positively charged protons from one another. Under certain circumstances, the repelling electromagnetic force becomes stronger than the nuclear force, and nucleons can be ejected from the nucleus, leaving behind a different element: nuclear decay resulting in nuclear transmutation.

The number of protons in the nucleus defines to what chemical element the atom belongs: for example, all copper atoms contain 29 protons. The number of neutrons defines the isotope of the element. The number of electrons influences the magnetic properties of an atom. Atoms can attach to one or more other atoms by chemical bonds to form chemical compounds such as molecules. The ability of atoms to associate and dissociate is responsible for most of the physical changes observed in nature and is the subject of the discipline of chemistry. — Wikipedia

Atoms (Eric Weisstein’s World of Physics, Wolfram Research)
Atom (Encyclopædia Britannica)


Atom (WolframAlpha)
Step by Step Chemistry (Wolfram Alpha)








Atomic Physics is the field of physics that studies atoms as an isolated system of electrons and an atomic nucleus. It is primarily concerned with the arrangement of electrons around the nucleus and the processes by which these arrangements change.

The term atomic physics can be associated with nuclear power and nuclear weapons, due to the synonymous use of atomic and nuclear in standard English. Physicists distinguish between atomic physics — which deals with the atom as a system consisting of a nucleus and electrons — and nuclear physics, which considers atomic nuclei alone. — Wikipedia

Nuclear physics is the field of physics that studies atomic nuclei and their constituents and interactions. Other forms of nuclear matter are also studied. Nuclear physics should not be confused with atomic physics, which studies the atom as a whole, including its electrons. — Wikipedia

How Can You See an Atom? – Legends of Chemistry (American Chemical Society)

Scientists perform world’s smallest MRI on single atoms (Brooks Hays, UPI)
World’s smallest MRI performed on single atoms (Institute for Basic Science,

Nuclear Physics (Wolfram Alpha)


Nuclear engineering is concerned with the application of breaking down atomic nuclei (fission) or of combining atomic nuclei (fusion), or with the application of other sub-atomic processes based on the principles of nuclear physics. In the sub-field of nuclear fission, it includes the design, interaction, and maintenance of systems and components like reactors and power plants. The field also includes the study of medical and other applications of radiation, particularly Ionizing radiation, nuclear safety,nuclear fuel, or other related technology (e.g., radioactive waste disposal). — Wikipedia

Nuclear power is the use of nuclear reactions to produce electricity. Nuclear power can be obtained from nuclear fission, nuclear decay and nuclear fusion reactions. Presently, the vast majority of electricity from nuclear power is produced by nuclear fission of uranium and plutonium in nuclear power plants. Nuclear decay processes are used in niche applications such as radioisotope thermoelectric generators in some space probes such as Voyager 2. Generating electricity from fusion power remains the focus of international research. — Wikipedia

Note: This is a 360° Video — press and hold to explore it!

Nuclear Power (Wolfram Alpha)



Atom Campaigns (Kickstarter)
Atom Campaigns (Indiegogo)


Atom Gifts (Zazzle)




Niels Bohr Archive (University of Copenhagen)
Niels Bohr Institute (University of Copenhagen)
Niels Bohr (Encyclopædia Britannica)
Neils Bohr (Wikipedia)


National Museum of Nuclear Science & History (Albuquerque, New Mexico)


Atomic Heritage Foundation (Official Site)
Atomic Heritage Foundation (YouTube Channel)
Atomic Heritage Foundation (Wikipedia)

The Nuclear Files Archive (Nuclear Age Peace Foundation)


Library of Congress # QC793 Nuclear Structure (UPenn Online Books)

WorldCat, Library of Congress, UPenn Online Books, Open Library




Nuclear Science (Contemporary Physics Project)

Atoms Cafe (PlayMada Games)

Atoms Around Us (Chem4Kids)
Nuclear Physics (Physics4Kids)

Atomic Structure (ChemMatters, American Chemical Society)
Nuclear Chemistry (ChemMatters, American Chemical Society)

Nuclear Science 101 (American Nuclear Society)
About Nuclear (American Nuclear Society)

Nuclear Engineering (American Society for Engineering Education)

MERLOT: Multimedia Educational Resource for Learning and Online Teaching
OER Commons: Open Educational Resources


General Chemistry I: Atoms, Molecules, and Bonding (MITx)

Chemistry Department Courses (MIT OpenCourseWare)
Chemistry Courses (edX)

Nuclear Science and Engineering Department (MIT OpenCourseWare)
Nuclear Sciences & Engineering (Course 22, MITx)

Nuclear Physics Courses (MIT OpenCourseWare)
Nuclear Engineering Courses (MIT OpenCourseWare)
Nuclear Materials Courses (MIT OpenCourseWare)

Nuclear Physics Courses (edX)



Nuclear Chemistry Careers (American Chemical Society)

Nuclear Engineers (CareerOneStop, U.S. Department of Labor)

Nuclear Engineer (US Bureau of Labor Statistics)


American Nuclear Society

Nuclear Age Peace Foundation


Nuclear News (American Nuclear Society)
Publications (American Nuclear Society)

Nuclear chemistry (Nature)
Nuclear Energy (
Atomic & Nuclear (Scientific American)
Nuclear Energy (NPR Archives)


Nuclear Energy (ISBNdb)


International Atomic Energy Agency

Office of Nuclear Energy (US Energy Department)
US Nuclear Regulatory Commission






Fusion (Tim Hunkin, The Rudiments of Wisdom Encyclopedia)
Radiation (Tim Hunkin, The Rudiments of Wisdom Encyclopedia)
Plutonium (Tim Hunkin, The Rudiments of Wisdom Encyclopedia)


The Man Who Hunts Hidden Radioactive Objects (Chris Baraniuk, BBC Future)

Visual Arts

Minute Physics (YouTube Playlist)


Each atom absorbs unique light,
Leaving lines that are black as the night.
This absorption occurs
At the bits it prefers,
So the rest of the spectrum stays bright. — DenmarK, absorption spectrum

OEDILF: The Omnificent English Dictionary In Limerick Form



Here are links to pages about closely related subjects.

Knowledge Realm


Law (Constant) Relativity
Force Gravity, Electromagnetism (Light, Color)
Matter (Microscope) Molecule, Atom (Periodic Table), Particle

Universe (Astronomical Instrument)
Galaxy Milky Way, Andromeda
Planetary System Star, Brown Dwarf, Planet, Moon

Our Neighborhood
Solar System Sun
Terrestrial Planet Mercury, Venus, Earth (Moon), Mars
Asteroid Belt Ceres, Vesta
Jovian Planet Jupiter, Saturn, Uranus, Neptune
Trans-Neptunian Object
Kuiper Belt Pluto, Haumea, Makemake
Scattered Disc Eris, Sedna, Planet X
Oort Cloud Etc. Scholz’s Star
Small Body Comet, Centaur, Asteroid


1.   The resources on this page are are organized by a classification scheme developed exclusively for Cosma.