A heterokaryon contains genetically different cell nuclei in a shared cytoplasm; cell fusion can create one.

Cell culture maintains biological cells outside their original organism.^[ref]

Glycosylation attaches carbohydrates to other molecules, including proteins and lipids.^[ref]

Peptide mapping compares separated peptides produced by cleavage of a protein. Matching patterns provide a chemical comparison beyond an intact protein's gel electrophoresis position.

Proteolysis cleaves peptide bonds in proteins; it differs from reduction of protein disulfide bonds.

The Rutherford model places a concentrated central electric charge inside a much larger chemical atom. In 1911, Ernest Rutherford argued that rare large deflections were predominantly single close encounters, rather than accumulated small deviations in a diffuse-charge atom. He treated the central electric charge as positive for calculation while noting that the scattering deductions did not establish its sign. He explicitly deferred atomic stability.^[ref]

The physical inference is a compact region generating a strong electric field, not a miniature hard wall. Most impact parameters miss that region, while a small subset give large deflections. The model supplies a way to infer interior structure from outgoing angles, a central use of scattering throughout atomic physics and nuclear physics. This inference does not by itself specify electron quantum states or explain spectral lines.

The repulsive model also sets a probing scale. At closest approach, conservation of energy with zero radial velocity givesFor the numerical model above, even a head-on projectile turns at $r_{\min }=2a=45.504\mathrm{fm}$. This is a calculated closest-approach distance, not a measured atomic nucleus radius. A point-charge fit can constrain how compact the scattering centre must be, but cannot directly resolve structure at distances the projectiles never reach.

The Rutherford differential cross-section must not be extrapolated to zero angle: the unscreened point model has a forward divergence, while real neutral chemical atoms exhibit the screening effect, finite beam acceptance, and accumulated small deflections. At very short separation, finite nuclear size and the nuclear force can change the interaction. Nuclear recoil changes the relation between laboratory and centre-of-mass angles for lighter targets; appreciable energy loss or repeated large deflections invalidates the simple thin-foil scattering counts. These limits delimit the calculation rather than supplying evidence for a nuclear contact collision in the early experiment.

Rutherford scattering is elastic scattering by a Coulomb potential. In the repulsive classical case, a positively charged projectile bends around a positively charged atomic nucleus. A close approach can turn it backwards without a contact collision. The calculation connects an invisible impact parameter with a measurable outgoing angle, then with a differential scattering cross-section and a detector count.

Assume a projectile of mass $m$ and electric charge $ze$, a stationary point atomic nucleus of electric charge $Ze$, and positive $z,Z$. Here $e$ is the elementary charge. Neglect nuclear recoil, electron screening effect, energy loss, and the nuclear force. With vacuum permittivity ${\epsilon }_0$, defineHere $E$ and $v$ are the incident kinetic energy and speed far from the atomic nucleus, $b\ge 0$ is the impact parameter, and $L$ is the magnitude of angular momentum. The nonrelativistic limit requires $v\ll c$, with $c$ the speed of light. The fixed-target approximation requires the target mass to be much greater than $m$. In SI, $\kappa$ has units $\mathrm{J}\mathrm{m}$, $E$ has units $\mathrm{J}$, and $b$ has units $\mathrm{m}$.

The Geiger-Marsden experiment probes the interior of a chemical atom by counting alpha particles deflected by metal foils. The surprising observation was a small population returning towards the incident side. Hans Geiger and Ernest Marsden performed the experiments under Ernest Rutherford's direction; Ernest Rutherford developed the Rutherford model in 1911. Observation and interpretation were distinct contributions.^[ref]

In 1909, a radon source and its radioactive decay products in a conical glass tube supplied alpha particles through a mica window towards a reflector. Lead shielded a fixed zinc sulfide screen from direct hits; an optical microscope resolved individual particle scintillations. Reflectors occupied the same position. Reflector-out counts tested air scattering; stacked gold foils and bare glass tested thickness and support contributions.^[ref]

Nearby source geometry blurred incidence angles. Extrapolating platinum counts uniformly over a hemisphere was an assumption, not a universal reflection probability.^[ref]

A change between electronic energy eigenstates of a chemical atom or another system.

Determining the column density of a cloud of chemical atoms from the attenuation of a probe beam of light; the measured image integrates through the cloud along the viewing direction.

A quantum electrodynamics correction separates hydrogen atom levels such as $2s_{1/2}$ and $2p_{1/2}$ that coincide in the simplest model.

The Lamb shift measures an energy level displacement beyond a specified Dirac equation reference. In the point-proton, infinite-nuclear-mass Coulomb potential model, $2S_{1/2}$ and $2P_{1/2}$ share principal quantum number $n=2$ and total electronic angular momentum $j=1/2$, hence energy, despite differing orbital angular momentum values.^[ref]

The Lamb-Retherford experiment converts a small energy difference into a large detector change: prepare a long-lived quantum state, transfer population into a short-lived one, and read the surviving beam.

A restriction on transitions between quantum states for a specified interaction. An electric-dipole selection rule is one example.

A Rydberg atom has an electron excited to a large principal quantum number $n$. In a hydrogen-like model, the binding energy scales as $1/n^2$ and the orbital size as $n^2$. The outer electron is therefore weakly bound and its motion extends far beyond the compact core.

These properties make Rydberg atoms sensitive to applied electric fields and useful for studying controllable interactions such as Rydberg blockade. A field ramp can ionize different excited states at different thresholds, enabling state-selective field ionization. Real multi-electron atoms depart from the simple hydrogen-like spectrum, especially when the outer electron penetrates the core.

A cold atom has a small motional kinetic energy relative to a stated experimental scale. Usually an ensemble is prepared with a narrow velocity distribution through laser cooling or evaporative cooling of atoms; the quoted temperature characterizes motion rather than requiring the atom's internal electronic state to be thermally equilibrated.

Slower motion increases the time available for interrogation in atomic clocks or atom interferometry. In a release experiment, a cloud's growth after the trap is switched off constrains its velocity spread. Distinguishing initial cloud size from expansion avoids mistaking tight confinement for low motional temperature.

An ultracold gas is a dilute gas cooled until atomic motion becomes slow enough that quantum mechanics is essential to its preparation, collisions or collective state. There is no single temperature boundary independent of particle mass and number density: the overlap of thermal matter waves depends on the thermal de Broglie wavelength relative to interparticle spacing.

Laser cooling can prepare a cold trapped sample, and evaporative cooling of atoms can further increase its phase-space density of an atomic gas. Expansion after releasing a trap gives information about the velocity distribution. A small measured cloud alone is insufficient evidence for low temperature, because spatial confinement also depends on the trapping potential energy.

In LS coupling, an atomic term symbol specifies total electronic spin $S$, total orbital angular momentum $L$, and total angular momentum $J$. The superscript $2S+1$ is the spin multiplicity; the letters $S,P,D,F,\dots$ denote $L=0,1,2,3,\dots$. Coupling gives $J=|L-S|,\dots ,L+S$, and a fixed-$J$ level has $2J+1$ magnetic substates before an external field resolves them. For example, ${}^{3}P$ has $S=L=1$ and levels $J=0,1,2$, containing $1+3+5=9$ states altogether. A term symbol does not itself specify the radial electron configuration.

Electrons partially shield an atomic nucleus electric charge. At large distances a neutral chemical atom therefore does not generate the unscreened Coulomb potential of its bare atomic nucleus.

An atom is monovalent in a specified bonding or conduction model when its relevant valence is one. In the simple metallic free electron model, one monovalent atom per primitive unit cell supplies one conduction electron per cell. Counting the two spin states then fills half of a single energy band at zero temperature.

A muonic atom has a negative muon bound in the Coulomb potential of an atomic nucleus. Its much larger reduced mass than in an electronic chemical atom greatly reduces the orbital scale, increasing sensitivity of transition energies to the nuclear charge radius and nuclear charge distribution. Measuring muonic X-ray transitions therefore complements electron scattering. Strong relativistic and nuclear-size effects in heavy systems require a treatment beyond a point-nucleus nonrelativistic Coulomb spectrum.

Orbital parallax infers a system's Euclidean distance by comparing the angular size of an orbit with its physical size. If an orbital fit gives a deprojected angular semimajor axis $\alpha$ in radians and spectroscopy gives the corresponding physical semimajor axis $a$, the small-angle approximation gives $d=a/\alpha$.

The scales must describe the same orbit. In a binary star, radial-velocity amplitudes, orbital period and eccentricity give physical projected orbital sizes; the astrometric geometry can supply the orbital inclination needed to remove that projection. The mass-distance degeneracy of an angular stellar orbit gives a worked example of why adding a physical velocity measurement changes what can be inferred.^[ref]