Atoms

Everything you have ever touched is mostly empty space wearing a very convincing disguise.

By Greg Goodwin · Published October 17, 2026 · Updated October 24, 2026

Cheat Sheet

Electron charge: 1.602 x 10^-19 coulomb, first measured by Robert Millikan in 1909-1910; a proton carries the same size of charge, but positive.
Weight gap: a proton or neutron is about 1,836 times heavier than an electron.
Size: Joseph Loschmidt estimated an atom's diameter in 1865 at roughly 10^-8 cm, or about 0.1 nanometer.
Carbon-12 has 6 protons and 6 neutrons; carbon-14 has 6 protons and 8 neutrons, and both behave as carbon.
Atomic mass unit: defined as 1/12 the mass of a carbon-12 atom, about 1.66 x 10^-24 gram.
Avogadro's number: about 6.022 x 10^23, the number of atoms in 12 grams of carbon-12.

The 60-Second Version

An atom is the smallest unit of matter that still has the characteristic properties of a chemical element, which makes it the basic building block of chemistry. Each atom has a tiny, dense nucleus made of positively charged protons and uncharged neutrons, surrounded by much lighter, negatively charged electrons. Opposite charges attract, and that electrical pull keeps the electrons around the nucleus. The number of protons, called the atomic number, determines the element: every atom with six protons is carbon, no matter what else is going on inside. Atoms of the same element can differ in their neutron counts, and those variants are called isotopes. The nucleus occupies only a very small part of the atom, so most of an atom is empty space. Our modern picture took shape in a burst of experiments between 1897 and 1913, and quantum mechanics later refined it further.

The Long Version

From Philosophy to Proof

The idea of atoms is about 2,000 years old, but for most of that time it was philosophy rather than evidence. In 1738 Daniel Bernoulli imagined gases as vast numbers of particles in chaotic motion, but the idea was largely ignored for more than a century. Joseph Loschmidt made the first modern estimates of atomic size in 1865 and put the diameter of an atom near 10^-8 cm. In 1908 Jean Perrin used Brownian motion, the jittering of tiny particles in water, to pin down Avogadro's number. Britannica's account notes that after Perrin, few scientists could disbelieve the existence of atoms.

The Electron Cracks the Atom Open

The old view held that an atom was a single indivisible lump. In 1897 English physicist J.J. Thomson showed that cathode rays were streams of negatively charged particles, which he called electrons. By comparing their mass to charge ratio with that of atoms, he found that an electron weighed only 1/1,836 as much as a hydrogen ion. The atom clearly had parts. Between 1909 and 1910 Robert Millikan watched charged oil drops hover in an electric field and found their charges were always whole-number multiples of one basic unit, 1.602 x 10^-19 coulomb. He won the 1923 Nobel Prize in Physics for the work.

Rutherford's Shock

Thomson imagined positive and negative charge spread evenly through the atom. In 1911 Ernest Rutherford overturned that model. His postdoctoral fellow Hans Geiger and student Ernest Marsden fired alpha particles at gold foil only 0.00004 cm thick. Most went straight through, but about one in 20,000 bounced off at 45 degrees or more. Rutherford later said it was as incredible as firing a 15-inch shell at tissue paper and having it come back to hit you. The only explanation was a tiny, massive, charged nucleus carrying most of the atom's mass, with electrons in the surrounding space.

Numbers That Define the Elements

In 1913 English physicist Henry Moseley measured the X-rays emitted by many elements and found a regular pattern tied to the charge on the nucleus, which he called the atomic number. It sorted the elements better than atomic weight did: cobalt is heavier than nickel, but cobalt has atomic number 27 and nickel 28. That also meant the periodic table had a definite number of slots to fill. Meanwhile, Thomson and later Francis Aston showed that many elements come as isotopes. Neon, for example, averages 909 neon-20 atoms, 88 neon-22, and 3 neon-21 in every 1,000, so an element's atomic weight is a weighted average. Moseley was killed in World War I.

Light, Quanta, and Bohr

Each element gives off a unique pattern of spectral lines; in 1859 Gustav Kirchhoff and Robert Bunsen even discovered cesium and rubidium by spotting new lines. Classical physics could not explain why those lines appear, and Britannica notes that only quantum mechanics could. Niels Bohr's quantum shell model, developed at the same time as Moseley's work, placed electrons in distinct energy levels. Rutherford's solar-system picture is a handy cartoon, but electrons actually follow the rules of quantum mechanics, which allow only certain energies and spins.

Why People Care

Atoms are the reason chemistry works: every material, from steel to your own cells, is a particular arrangement of them. Understanding atoms also explains isotopes, radioactivity, spectra, and the periodic table, and it is the foundation for chemistry, materials science, and nuclear physics.

Glossary

Atomic number: The number of protons in a nucleus, which sets which element an atom is.
Isotope: One of several forms of an element with the same number of protons but different numbers of neutrons.
Nucleus: The tiny, dense center of an atom made of protons and neutrons, holding most of its mass.
Electron: A light, negatively charged particle that surrounds the nucleus.
Mass number: The total count of protons plus neutrons in a nucleus, as in carbon-12.

Go Deeper

More to Explore

🎲 Take Me Somewhere Random