atomThe building block of matter: a nucleus plus electrons. Any atom with 29 protons is copper. The kind of atom is set by its protons.
atomic numberThe proton count of an element; hydrogen's is 1. Each oxygen atom has 8 protons, so the atomic number of oxygen is 8.
electric chargeWhat a proton has as plus and an electron as minus. Two charges of one kind push apart, and a plus and a minus pull together.
electronThe lightweight particle with a charge of minus one. An electron is not made of smaller particles. Its negative charge holds it to the positive nucleus.
electron shellAn orbit-like zone that electrons fill, named K, L, M. Two electrons fill the first shell, and the second has room for eight.
elementOne species of atom, such as iron, neon or carbon. What sets one element apart from another is the number of protons in its atoms.
ionAn atom or molecule whose charges do not cancel out. A cation is an ion with a positive charge, and an anion is an ion with a negative charge.
Show all 36 terms (29 more)
Parts of an atom, continued
isotopeOne of an element's versions, which differ in neutrons. Carbon-12 has 6 neutrons, carbon-13 has 7 and carbon-14 has 8: three isotopes of carbon.
mass numberThe sum of nuclear particles: the 14 of carbon-14. Carbon-14 has 6 protons and 8 neutrons. Carbon-12 has 6 of each.
neutronThe neutral partner of the proton in a nucleus. A neutron has slightly more mass than a proton. The nucleus of ordinary hydrogen has none.
nucleusAn atom's small, dense core. It takes up almost none of the atom's space and holds more than 99.9 percent of its mass.
protonA nuclear particle with a charge of plus one. Every atom has at least one. A proton has about 1,836 times the mass of an electron.
Atomic structure 12
atomic massThe mass of one single atom. Nearly all of it comes from the atom's protons and neutrons.
atomic mass unitA unit of mass: one twelfth of an atom of carbon-12. Its other name is the dalton. A proton or a neutron has a mass of about one.
Bohr modelAn early picture of the atom, with electrons in orbits. It was developed from 1911 to 1918 and looks like a small solar system. Later models replaced it.
electron cloudThe haze of electrons that surrounds a nucleus. It is drawn as a cloud because an electron has no marked path, only regions where it is likely to be found.
electron configurationThe way an atom's electrons are arranged. The electron configuration of neon is written 1s2 2s2 2p6.
energy levelOne of the fixed amounts of energy an electron may have. An electron in an atom cannot have just any amount of energy, only certain ones.
excited stateAny state with more energy than the lowest one. The step up in energy is called excitation.
ground stateThe condition of an atom holding the least energy it can. Every other state has more energy than this one.
nucleonEither kind of particle that makes up a nucleus. Nucleon is one word for both the proton and the neutron.
quarkA particle that protons and neutrons are built from. Two up quarks with one down quark make a proton. One up with two down make a neutron.
subatomic particleAn electron or anything else tinier than an atom. Protons, neutrons and electrons are subatomic particles.
valence electronAn electron of the last shell, the kind that forms bonds. Last here means outermost: the shell farthest from the nucleus.
Matter, models and the nucleus 12
atomic theoryThe idea that all matter is made of atoms. At first the atom was thought to be a particle that could not be divided.
chemical bondA pull that links atoms into molecules or crystals. Atoms may bond by sharing electrons, or by the pull between ions of opposite charge.
chemical symbolAn element's short form, written in one or two letters. H stands for hydrogen. The first letter of a symbol is a capital.
elementary particleA particle that is not made of anything smaller. Electrons and quarks are elementary particles. Protons and neutrons are not.
gold foil experimentThe 1909 test that led to the finding of the nucleus. Hans Geiger and Ernest Marsden did the work under Ernest Rutherford, who explained it in 1911.
half-lifeHow long half of a batch of unstable nuclei lasts. The half-life of carbon-14 is about 5,730 years.
matterEverything with mass and volume, solid, liquid or gas. Anything you can touch is made of atoms.
neutral atomAn atom whose plus and minus charges balance exactly. It has as many electrons as protons.
periodic tableThe grid of periods and groups that lists every element. The elements stand in order of atomic number. A period is a row and a group is a column.
radioactive decayThe breakdown over time of nuclei that are not stable. The nucleus loses energy by giving off radiation. Alpha, beta and gamma decay are three common types.
radioisotopeAn unstable isotope, one that gives off radiation. Every isotope made in a laboratory is a radioisotope.
strong forceThe pull that keeps a nucleus from flying apart. It holds the protons and neutrons together, and it is the strongest known fundamental force.
How this list was compiled: Term list chosen for this site on 2026-10-11. Each meaning is written in our own words and was checked that day against the English Wikipedia article on the term (https://en.wikipedia.org/), whose text is under the Creative Commons Attribution-ShareAlike license: Atom, Atomic nucleus, Proton, Neutron, Electron, Electron shell, Atomic number, Mass number, Isotope, Chemical element, Ion, Electric charge, Subatomic particle, Nucleon, Quark, Atomic orbital, Energy level, Valence electron, Ground state, Excited state, Bohr model, Electron configuration, Atomic mass, Dalton (unit), Matter, Chemical bond, Periodic table, Chemical symbol, Hydrogen, Isotopes of hydrogen, Tritium, Radioactive decay, History of atomic theory, Rutherford scattering experiments, Strong interaction, Half-life, Radionuclide and Elementary particle. The nucleus, the proton, the neutron, the quark, isotopes and radioactive decay were checked a second time against the DOE Explains pages of the U.S. Department of Energy Office of Science (https://www.energy.gov/science/doe-explainsnuclei and its pages on protons, neutrons, isotopes, and quarks and gluons). The fact after each meaning is from the same pages. Licence: own work
How it works
This atom vocabulary worksheet lists nucleus, proton, neutron, electron, isotope, atomic number and six more atom vocabulary words down the left, with their meanings in another order down the right. A key follows as the last page.
Read the full guide
An atomic structure vocabulary worksheet
Set opens on Parts of an atom. Atomic structure goes further into the structure of an atom: subatomic particle, nucleon, quark, electron cloud, energy level, valence electron, ground state, excited state, Bohr model, electron configuration, atomic mass and atomic mass unit.
Matter, models and the nucleus
The third set of atoms vocabulary has matter, atomic theory, gold foil experiment, chemical bond, periodic table, chemical symbol, strong force, half-life, radioisotope, elementary particle, neutral atom and radioactive decay.
Molecule, compound, cation and anion are left to the chemistry flashcards.
Questions and answers
What are the three main parts of an atom?
Protons, neutrons and electrons. Protons, which carry a positive charge, and neutrons, which carry none, make up the nucleus at the center. Electrons carry a negative charge and surround the nucleus.
What is the difference between atomic number and mass number?
The atomic number is the number of protons in the nucleus, and it tells which element the atom is. The mass number is the protons and neutrons counted together. Carbon-14 has atomic number 6 and mass number 14.
What is an isotope?
Isotopes are atoms of one element with different numbers of neutrons. Hydrogen has three in nature, each with one proton: protium has no neutrons, deuterium one and tritium two. Protium and deuterium are stable, and tritium is radioactive.
What holds an atom together?
Two attractions. The negative electrons are held to the positive nucleus by the pull between opposite charges. Inside the nucleus, the strong force holds the protons and neutrons together, although the protons push each other apart.