Plural of “nucleus”

The plural of nucleus is nuclei (some dictionaries also accept nucleuses).

Why

"Nucleus" descends from Latin for "a little nut" or "kernel," a diminutive of "nux" (nut), and was adopted into scientific English across the 18th and 19th centuries to describe the central core of a cell, an atom, or a comet. As an ordinary Latin -us noun, it swaps that ending for -i in the plural, giving "nuclei."

The word's scientific range is genuinely broad — cell biology, atomic physics, and even brain anatomy (a "nucleus" as a cluster of neurons) all use "nucleus/nuclei" identically, and because the word lives almost entirely within specialist scientific writing, it has shown far less pressure to regularize toward "nucleuses" than more everyday Latin borrowings like "cactus." The word's reach into brain anatomy specifically deserves its own note: a "nucleus" in neuroscience refers to a cluster of neuron cell bodies serving a shared function (the caudate nucleus, the accumbens nucleus), a sense distinct from both the cellular and atomic meanings but pluralizing identically as "nuclei" — meaning a neuroscientist, a cell biologist, and a physicist all use the exact same irregular plural despite describing entirely different physical structures at wildly different scales. This consistency across such different scientific fields is part of why "nuclei" feels so thoroughly entrenched compared to more everyday Latin plurals that show real regional or registerbased variation.

Usage note

"Nucleuses" does exist and is listed in some dictionaries, but it reads as distinctly unusual to anyone with a science background, since "nuclei" is essentially universal in biology and physics writing at every level from introductory textbooks to research journals.

The adjective "nuclear" is a good reminder that irregularity doesn't travel from noun to adjective automatically — it's completely regular, inflecting for nothing at all, the way virtually every English adjective built on an irregular Latin noun turns out to be. Popular science writing aimed at general audiences occasionally simplifies by avoiding the plural entirely, rephrasing around it ("each atom's central core") specifically to sidestep any reader confusion over an unfamiliar Latin plural, a common strategy science communicators use across several of the Latin -us/-i words discussed on this site. Some introductory science textbooks now include a brief note explaining the Latin plural specifically because so many students otherwise assume 'nuclei' is an unrelated vocabulary word rather than simply the plural of a term they already know.

Common real mistakes

A genuine, common student error is treating "nuclei" as if it named a different structure entirely rather than recognizing it as simply the plural of "nucleus" — this usually resolves quickly once a student encounters the word in both singular and plural form within the same lesson. A related and genuinely common student mistake is assuming every atom has multiple nuclei, when in fact each atom has exactly one nucleus; the plural only becomes relevant when discussing multiple atoms or multiple cells together, not multiple structures within a single atom.

The wider pattern

Physics, cell biology, and even neuroscience all borrow the identical Latin -us/-i pattern seen in "cactus/cacti," "radius/radii," and "alumnus/alumni" — a reminder that "nuclei" has stayed so stable specifically because the word's use has remained concentrated in technical, specialist writing across every one of those fields.

Examples

• Each cell in the tissue sample had a clearly visible nucleus.
• The atoms' nuclei were bombarded with high-energy particles.
• A single nucleus controls most of the cell's genetic activity.
• Researchers compared nuclei across several different cell types.
• The comet's nucleus was smaller than earlier estimates suggested.
• Multiple nuclei formed after the unusual cell division.
• The nucleus of the atom contains protons and neutrons.
• Scientists imaged the nuclei using an electron microscope.
• Each neuron cluster, or nucleus, serves a distinct brain function.
• The cell's nucleus divided cleanly during mitosis.
• The professor drew a simplified diagram showing electrons orbiting the nucleus.
• Both nuclei fused briefly during the high-energy collision experiment.
• The unstable nucleus decayed within milliseconds of formation.
• Researchers mapped how signals traveled between different brain nuclei.