Στο εξώφυλλο: Βασικό διάγραμμα ατόμου με πρωτόνια, νετρόνια & ηλεκτρόνια. KTSDESIGN/SCIENCE PHOTO LIBRARY / Getty Images
by Anne Marie Helmenstine, Ph.D.
Updated June 06, 2017
An atom is the defining structure of an element, which cannot be broken by any chemical means. A typical atom consists of a nucleus of positively-charged protons and electrically neutral neutrons with negatively-charged electrons orbiting this nucleus. However, an atom can consist of a single proton (i.e., the protium isotope of hydrogen) as a nucleus. The number of protons defines the identity of an atom or its element.
The size of an atom depends on how many protons and neutrons it has, as well as whether or not it has electrons. A typical atom size is around 100 picometers or about one ten-billionth of a meter. Most of the volume is empty space, with regions where electrons may be found. Small atoms tend to be spherically symmetrical, but this is not always true of larger atoms. Contrary to most diagrams of atoms, electrons do not always orbit the nucleus in circles.
Atoms can range in mass from 1.67 x 10-27 kg (for hydrogen) to 4.52 x 10-25 kg for superheavy radioactive nuclei. The mass is almost entirely due to protons and neutrons, as electrons contribute negligible mass to an atom.
An atom that has an equal number of protons and electron has no net electrical charge. An imbalance in the numbers of protons and electrons forms an atomic ion. So, atoms may be neutral, positive, or negative.
The concept that matter might be made of small units has been around since ancient Greece and India.
In fact, the word «atom» was coined in Ancient Greece. However, the existence of atoms was not proven until John Dalton’s experiments in the early 1800s. In the 20th century, it became possible to «see» individual atoms using scanning tunneling microscopy.
While it’s believed electrons formed in the very early stages of the Big Bang formation of the universe, atomic nuclei did not form until perhaps 3 minutes after the explosion.
At present, the most common type of atom in the universe is hydrogen, although over time, increasing amounts of helium and oxygen will exist, likely overtaking hydrogen in abundance.
Most of the matter encountered in the universe is made from atoms with positive protons, neutral neutrons, and negative electrons. However, there exists an antimatter particle for electrons and protons with opposite electrical charges. Positrons are positive electrons, while antiprotons are negative protons. Theoretically, antimatter atoms might exist or be made. The antimatter equivalent to a hydrogen atom (antihydrogen) was produced at CERN in Geneva in 1996. If a regular atom and an anti-atom were to encounter each other, they would annihilate each other, while releasing considerable energy.
Exotic atoms are also possible, in which a proton, neutron, or electron is replaced by another particle. For example, an electron could be replaced with a muon to form a muoinic atom. These types of atoms have not been observed in nature, yet may be produced in a laboratory.
Examples of substances that are not atoms include water (H2O), table salt (NaCl), and ozone (O3). Basically, any material with a composition that includes more than one element symbol or that has a subscript following an element symbol is a molecule or compound and not an atom.