Atomic Theory


alpha particle

particle identical to a helium ion (He2+) that is emitted during the decay of radioactive elements

angstrom (Å)

unit of length equal to 10–10 meter


atom or group of atoms that has more electrons than protons, giving it an overall negative charge


smallest particle of an element that has the properties of that element

atomic mass unit (amu)

mass of a single proton or neutron, equal to 1 gram per mole (g/mol), defined as one-twelfth the mass of a carbon-12 atom

atomic number (Z)

number of protons in each atom of an element

atomic weight

average mass of all the isotopes of an element, based on the relative abundance of each isotope

Bohr radius

distance between the nucleus and the electron in a hydrogen atom

cathode ray

beam of electrons emitted from a negatively charged conducting plate in a vacuum chamber containing very little gas


atom or group of atoms that has more protons than electrons, giving it an overall positive charge

chemical symbol

letter or letters used to represent an element


negatively charged subatomic particle

electron cloud

arrangement of electrons moving around an atomic nucleus based on probabilities of their locations

fundamental unit of charge (e)

charge of an electron, 1.6021766×1019C1.6021766\times{10}^{-19}\;\rm{C} (coulomb)


one of two or more atoms of an element that have the same number of protons but different numbers of neutrons

law of conservation of mass

law that states that matter is neither created nor destroyed during a chemical reaction

law of definite proportions

law that states that compounds have the same proportions of masses of their constituent elements regardless of how small they are broken down

law of multiple proportions

law that states that when two elements combine to form more than one compound, the mass of one element combines with a fixed mass of the other in a ratio of small whole numbers

mass number (A)

sum of the number of protons and neutrons in a nucleus


subatomic particle that has a neutral charge in the nucleus of an atom


positively charged center of an atom containing protons and neutrons


region in which an electron has a high probability of being located. Orbitals are described by the quantum numbers s, p, d, and f, which differ from one another by their shapes.


positively charged subatomic particle in the nucleus of an atom


spontaneously emitting energetic particles or radiation