Definition · Plain-language
Atomic structure
Atomic structure describes how an atom is built — a dense central nucleus of protons and neutrons surrounded by electrons.
The step most authors miss
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The three subatomic particles
An atom is built from three kinds of particle. Protons carry a positive charge and sit in the nucleus. Neutrons carry no charge and also sit in the nucleus, adding mass and helping to hold it together. Electrons carry a negative charge and occupy the space around the nucleus. Protons and neutrons have almost equal mass and account for nearly all of the atom’s mass; electrons are roughly two thousand times lighter. Because each proton’s positive charge is balanced by an electron’s negative charge, a neutral atom has equal numbers of both.
The nucleus and the electron cloud
Almost all of an atom’s mass is concentrated in its nucleus, which is extraordinarily small compared with the atom as a whole — if an atom were the size of a stadium, the nucleus would be a pea at its centre. The rest of the atom is the region where electrons are found, arranged in energy levels or shells. Atoms are therefore mostly empty space. Electrons are not in fixed orbits but occupy orbitals, regions where they are most likely to be found, an idea from quantum mechanics that replaced the older planetary picture.
Atomic number, mass number and isotopes
Two numbers summarise an atom. The atomic number is the number of protons; it defines which element the atom is, since every atom of an element has the same proton count. The mass number is the total number of protons and neutrons in the nucleus. Atoms of the same element always share the atomic number but can differ in neutron count, and these variants are called isotopes — they have the same chemistry but different masses. The relative atomic mass on the periodic table averages an element’s naturally occurring isotopes.
Key facts
At a glance
- Definition: how an atom is built from a nucleus and surrounding electrons
- Nucleus: contains protons (positive) and neutrons (neutral)
- Electrons: negatively charged, occupy shells around the nucleus
- Atomic number: the number of protons, defining the element
- Mass number: the total of protons and neutrons
- Neutral atom: equal numbers of protons and electrons
Common misconceptions
What people often get wrong
Often heard: An atom is a solid, filled-in ball of matter.
Actually: An atom is mostly empty space. Nearly all its mass sits in a tiny central nucleus, with electrons occupying the relatively vast space around it.
Often heard: The number of neutrons defines which element an atom is.
Actually: The atomic number — the number of protons — defines the element. Atoms of the same element can have different numbers of neutrons; these are isotopes.
Often heard: Electrons orbit the nucleus in neat circles like planets around the Sun.
Actually: Electrons occupy orbitals, regions of probability described by quantum mechanics, rather than fixed planetary orbits. The planetary model is a simplified teaching picture.
Going deeper
