Atom

The atom is the physical system that acts as the building block of matter. Outside of exotic or extreme physics, all matter is composed of atoms, either individually or bound together to form molecules, crystals or other forms of matter.

An atom is composed of two parts: a central atomic nucleus and an electron cloud surrounding it.

  1. The atomic nucleus is the center of the system. It is an approximately spherical object composed protons and neutrons bound to each other through strong interaction. It is positively charged and has a radius of approximately ∼10−13\sim10^{-13} cm or ∼1\sim1 fm.
  2. The electron cloud surrounds the nucleus. It is a collection of one or more negatively charged electrons bound by electromagnetic interaction to the nucleus. The radius of the electron cloud depends on the amount of electrons and their energy level, but roughly speaking, it is on the order of ∼10−8\sim10^{-8} cm or ∼0.1\sim0.1 nm. The Angstrom is also commonly used, 1 A˚≡0.1 nm1\ \mathring{\mathrm{A}}\equiv 0.1\text{ nm}.

This shape was proven empirically in 1911 by the Rutherford experiment.

Notation

Atoms conventionally use then notation \ceZAXN\ce{^{A}_{Z}X_{N}}, where:

  • X\mathrm{X} is the chemical symbol.
  • ZZ is the number of protons, called the atomic number or nuclear charge. In a neutral atom, it is also the number of electrons.
  • AA is the atomic mass number. It is the integer closest to the ratio of the mass of the nucleus and the atomic mass unit. Though not the definition, in practice A=Z+NA=Z+N.
  • NN is the number of neutrons. It is equal to A−ZA - Z. This is typically not written out explicitly.

For example, uranium (at least, the most common isotope) is written as \ce92238U146\ce{_{92}^{238}U_{146}}, which means that uranium has a mass of (approximately) 238 atomic masses, has 92 protons, and 146 neutrons. More commonly, it's written \ce92238U\ce{^{238}_{92}U} without NN at the bottom right.