Nucleus (Chemistry)

Summary: The nucleus is the tiny, dense, positively charged central region of an atom, containing protons and neutrons and accounting for nearly all of the atom’s mass. Tags: igcse chemistry Created: 2026-07-18

The nucleus is the central core of an atom, occupying only a minute fraction of the atom’s total volume yet containing over 99.9% of its mass. It is composed of two types of subatomic particles — protons (which carry a relative charge of +1) and neutrons (which carry no charge) — collectively referred to as nucleons. The overall charge of the nucleus is therefore positive, due entirely to the protons present, and this positive charge is what holds the negatively charged electrons in their shells around the nucleus via electrostatic attraction. The particles in the nucleus are bound together by the strong nuclear force, one of the fundamental forces of nature, which acts over the extremely short distances within the nucleus to overcome the electrostatic repulsion between protons. The size of the nucleus is approximately 10,000 to 100,000 times smaller than the whole atom, making it an extraordinarily dense concentration of mass at the atomic core.

The Nucleus

Composition and Charge

The nucleus contains two types of nucleon:

• Protons: Positively charged particles with relative charge +1 and relative mass 1. The number of protons in the nucleus is the atomic number (Z), which defines the identity of the Element.
• Neutrons: Electrically neutral particles with relative charge 0 and relative mass 1. Neutrons provide nuclear stability by contributing to the strong nuclear force without adding electrostatic repulsion.

Because neutrons carry no charge, the overall charge of the nucleus is equal to the number of protons multiplied by the elementary charge (+1 per proton). For example, a carbon nucleus contains 6 protons and therefore carries a charge of +6.

Mass and Density

The mass of the nucleus is essentially the mass of the atom, since electrons have a negligible mass (approximately 1/1836 of a Proton). The mass number (A) is the total number of nucleons: A = number of protons + number of neutrons. The density of nuclear matter is extraordinarily high — approximately 10^14 g/cm^3, or about 100 trillion times denser than water. This extreme density arises because nearly all the atom’s mass is packed into a volume that is roughly 10^-15 m in radius.

Nuclear Stability

For a nucleus to be stable, the ratio of neutrons to protons must fall within a certain range. In lighter elements (atomic numbers up to about 20), stable nuclei tend to have roughly equal numbers of protons and neutrons (N/Z ratio approximately 1). For heavier elements, the stable ratio increases, with more neutrons than protons needed to dilute the electrostatic repulsion between the increasing number of positively charged protons. When the N/Z ratio is outside the stable range, the nucleus may be radioactive and undergo nuclear decay. Isotopes of an element differ in the number of neutrons in the nucleus, which is why some isotopes are stable and others are radioactive.

Nuclear Size Relative to the Atom

If an atom were scaled up to the size of a football stadium (roughly 100 m in diameter), the nucleus would be approximately the size of a pea (about 1 cm in diameter). Alternatively, if the nucleus were the size of a marble (1 cm), the atom would be about 100 m across. The vast majority of an atom’s volume is therefore empty space, occupied only by the electrons in their shells.

Sources

• Cambridge IGCSE Chemistry 0620 Syllabus (2023-2025) — Section 2.2, Cambridge Assessment International Education

Common Misconceptions