The dense central part of an atom, containing protons and neutrons, is called the nucleus.
The nucleus occupies only a tiny fraction of an atom’s volume, yet it contains nearly all of the atom’s mass. Protons carry positive electric charge, while neutrons have no net charge. These particles are held together by the strong nuclear force, which acts over extremely short distances and counteracts the electrical repulsion between protons.
Electrons do not reside inside the nucleus. They occupy quantum states around it, often described as an electron cloud rather than fixed planetary-style orbits. This is why “orbital” and “shell” are related atomic terms but are not names for the atom’s central core.
The number of protons determines an element’s identity: every carbon atom has six protons, for example. Atoms of the same element can have different numbers of neutrons; these variants are called isotopes. In ordinary atoms, the number of electrons equals the number of protons, but losing or gaining electrons creates an ion without changing the nucleus into a different element.