Atom

An atom is the smallest piece of a chemical element that still behaves like that element. Every atom has two parts: a tiny, heavy Atomic nucleus built from Protons and Neutrons, wrapped in a cloud of Electrons. A neutral atom carries exactly as many electrons as protons, so its total Electric charge is zero.

Almost all of an atom's mass is in the nucleus. Almost all of its volume is empty space where the electrons live.

What it is made of

part	how many in a neutral atom	charge	where it sits
Proton	Z — the atomic number	+1 e each	nucleus
Neutron	A − Z, where A is the mass number	0	nucleus
Electron	Z	−1 e each	cloud around the nucleus

Three counts, three different meanings:

• Change Z and you have a different element. Z is the element.
• Change the neutron count and you have a different Isotope of the same element. Same chemistry, different mass and different nuclear stability.
• Change the electron count and you have an ion — an atom with a net charge.

Scale: mostly nothing

quantity	value	source
Bohr radius a₀ (hydrogen's size scale)	5.291 772 105 44 × 10⁻¹¹ m	CODATA 2022
proton rms charge radius	8.407 5 × 10⁻¹⁶ m	CODATA 2022
ratio of the two	≈ 62 900	the two rows above
proton-to-electron mass ratio	1836.152 673 426	CODATA 2022
atomic mass constant u	1.660 539 068 92 × 10⁻²⁷ kg	CODATA 2022

Two ways to feel those numbers:

• Blow a hydrogen atom up until its proton is a 1 cm marble. The electron cloud around it is then about 630 m across — roughly six city blocks. That is one marble in the middle of a neighbourhood, and nothing else.
• In that same hydrogen atom, the electron carries about 0.054 % of the mass. The proton carries the other 99.95 %.

How many kinds there are

NIST's table of atomic weights covers 118 elements, from hydrogen (Z = 1) to oganesson (Z = 118). It lists 354 nuclides in all and gives a measured natural abundance for 288 of them — those are the nuclides you can find on Earth without building them first. The remaining ones only exist in reactors, accelerators, or stars.

How we worked this out

1. 1897 — J. J. Thomson finds the Electron, the first proof that atoms have parts.
2. 1909–1911 — Geiger and Marsden fire alpha particles at gold foil. Most pass straight through; a few bounce almost straight back. Rutherford announces the nucleus in 1911: the positive charge and nearly all the mass are packed into a speck at the centre.
3. 1913 — Bohr explains why atoms emit sharp spectral lines, by allowing electrons only certain orbits.
4. 1932 — Chadwick finds the Neutron, which explains why isotopes of one element differ in mass.
5. 1920s onward — quantum mechanics replaces orbits with orbitals: clouds of probability, not paths.

Where to read next

• Atomic nucleus — the speck at the centre, and the force that holds it together
• Proton and Neutron — the two nucleons, and what they are made of
• Isotope — same element, different neutron count
• Quark — one level further down