Columbia Encyclopedia

Search results

302 results found

exclusion principle

(Encyclopedia)exclusion principle, physical principle enunciated by Wolfgang Pauli in 1925 stating that no two electrons in an atom can occupy the same energy state simultaneously. The energy states, or levels, in ...

CAT scan

(Encyclopedia)CAT scan kăt [key] [computerized axial tomography], X-ray technique that allows relatively safe, painless, and rapid diagnosis in previously inaccessible areas of the body; also called CT scan. An X-...

meson

(Encyclopedia)meson mēˈzŏn [key] [Gr.,=middle (i.e., middleweight)], class of elementary particles whose masses are generally between those of the lepton class of lighter particles and those of the baryon class ...

cyanobacteria

(Encyclopedia)cyanobacteria sīˌənōbăktĭrˈēə, sī-ănˌō– [key] or blue-green algae, photosynthetic bacteria that contain chlorophyll. For many years they were classified in the plant kingdom along with ...

Compton effect

(Encyclopedia)Compton effect [for A. H. Compton], increase in the wavelengths of X rays and gamma rays when they collide with and are scattered from loosely bound electrons in matter. This effect provides strong ve...

electronegativity

(Encyclopedia)electronegativity ĭlĕkˌtrōnĕgətĭvˈətē [key], in chemistry, tendency for an atom to attract a pair of electrons that it shares with another atom (see chemical bond). For example, the molecule...

Nilsson, Lennart

(Encyclopedia)Nilsson, Lennart lĕnˈnärt nĭlˈsôn [key], 1922–2017, Swedish photographer who made pioneering microphotographs of embryos and the human body. Nilsson began his career as a photojournalist, chro...

radiochemistry

(Encyclopedia)radiochemistry, chemistry of radioactive substances (see radioactivity). Radioactive isotopes are very useful as tracers to study the mechanisms of complex organic reactions, since even minute amounts...

Langmuir, Irving

(Encyclopedia)Langmuir, Irving lăngˈmyo͞or [key], 1881–1957, American chemist, b. Brooklyn, N.Y. Associated (1909–50) with the research laboratory of the General Electric Company, he introduced atomic-hydrog...

semiconductor

(Encyclopedia)CE5 N-type and P-type crystals: In a pure silicon crystal each silicon atom (Si) is surrounded by four valence electrons. In the N-doped semiconductor, illustrated in the left-hand figure, some sil...