In philosophy
The basic idea that matter is made up of tiny, indivisible particles appears in many ancient cultures such as those of Greece and India. The word atom is derived from the ancient Greek word

Nuclear properties
By definition, any two atoms with an identical number of protons in their nuclei belong to the same chemical element. Atoms with equal numbers of protons but a different number of neutrons are different isotopes of the

Baryonic matter forms about 4% of the total energy density of the observable universe, with an average density of about 0.25 particles/m (mostly protons and electrons). Within a galaxy

• Oliver Manuel (2001). Origin of Elements in the Solar System: Implications of Post-1957 Observations. Springer. ISBN 978-0-306-46562-8. OCLC 228374906.
• Andrew G. van Melsen (2004) [1952]. From Atomos to Atom: The History of the Concept Atom. Translated by Henry J.

Subatomic particles
Though the word atom originally denoted a particle that cannot be cut into smaller particles, in modern scientific usage the atom is composed of various subatomic particles. The constituent particles of an atom are the

While atoms are too small to be seen, devices such as the scanning tunneling microscope (STM) enable their visualization at the surfaces of solids. The microscope uses the quantum tunneling phenomenon, which allows particles to pass through a barrier that

• History of quantum mechanics
• Infinite divisibility
• List of basic chemistry topics