A nucleus can be any central part of something that gathers other parts around it. It comes from the Latin “nuc,“ meaning “kernel.” So a popular person with a lot of friends could be the nucleus of their friend group.
But in science, the nucleus can refer to the center of many different things. In chemistry, the nucleus is the central core of an atom. It’s the part that contains most of the atom’s mass. In cell biology, nucleus usually refers to a structure inside a cell that contains the cell’s genetic information. In neuroscience, however, the term nucleus refers to any cluster of brain cells that have the same connections and functions. And in astronomy, a nucleus refers to the rocky body of a comet.
So when you see the word nucleus, look for the context around it. Is it in space? In a cell? In the brain? And then, wherever you are, look for the center.
In a sentence
Every element has its own atomic number — the number of positively charged particles in its nucleus.