Definition · Plain-language
Atom
An atom is the smallest particle of a chemical element that still has the properties of that element, consisting of a dense nucleus surrounded by electrons.
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The structure of an atom
An atom is made of three kinds of particle. Protons carry a positive electric charge and neutrons carry none; together they sit in the nucleus, a tiny region at the centre that holds almost all the atom’s mass. Around the nucleus move electrons, which carry a negative charge and occupy a surrounding cloud of allowed energy levels. An atom is normally electrically neutral because it has equal numbers of protons and electrons. Most of an atom is empty space: if the nucleus were the size of a pea, the electrons would orbit hundreds of metres away.
What makes each element different
The identity of an element is set entirely by its atomic number — the number of protons in the nucleus. Hydrogen has one proton, carbon has six, oxygen has eight, and so on through the periodic table. Atoms of the same element with different numbers of neutrons are called isotopes; they behave almost identically chemically but differ in mass, and some are radioactive. Changing the number of protons, by contrast, changes the element itself, which is what happens in nuclear reactions.
How atoms join together
Atoms rarely stay alone. They bond with one another by sharing or transferring electrons to form molecules and compounds, which is the basis of all chemistry. Two hydrogen atoms and one oxygen atom bond to make a water molecule; vast networks of carbon atoms form diamond. The electrons in the outer energy levels do the bonding, while the nucleus stays unchanged. This is why ordinary chemical reactions rearrange atoms but never create or destroy them.
Key facts
At a glance
- Definition: the smallest unit of an element that keeps its chemical identity
- Nucleus: a dense centre of protons (positive) and neutrons (neutral)
- Electrons: negatively charged particles surrounding the nucleus
- Atomic number: the number of protons, which defines the element
- Neutral atom: equal numbers of protons and electrons
- Isotopes: atoms of one element with differing numbers of neutrons
Common misconceptions
What people often get wrong
Often heard: An atom is a solid ball that cannot be divided.
Actually: Atoms are mostly empty space and are made of smaller particles — protons, neutrons and electrons. The word means "indivisible", but atoms can be split in nuclear reactions, releasing energy.
Often heard: Electrons orbit the nucleus like planets around the Sun.
Actually: Electrons do not follow fixed circular paths. Quantum physics describes them as occupying regions of probability — orbitals — where they are likely to be found, not neat planetary orbits.
Often heard: The number of neutrons decides which element an atom is.
Actually: It is the number of protons (the atomic number) that defines the element. Atoms with the same protons but different neutrons are isotopes of the same element, not different elements.
Going deeper
