Isobars are atoms of different chemical element

Chemical element

A chemical element is a pure chemical substance consisting of one type of atom distinguished by its atomic number, which is the number of protons in its nucleus. Familiar examples of elements include carbon, oxygen, aluminum, iron, copper, gold, mercury, and lead.As of November 2011, 118 elements...

s that have the same number of nucleons. Correspondingly, isobars differ in atomic number

Atomic number

In chemistry and physics, the atomic number is the number of protons found in the nucleus of an atom and therefore identical to the charge number of the nucleus. It is conventionally represented by the symbol Z. The atomic number uniquely identifies a chemical element...

 (or number of protons) but not in mass number

Mass number

The mass number , also called atomic mass number or nucleon number, is the total number of protons and neutrons in an atomic nucleus. Because protons and neutrons both are baryons, the mass number A is identical with the baryon number B as of the nucleus as of the whole atom or ion...

. An example of a series of isobars would be ⁴⁰S, ⁴⁰Cl, ⁴⁰Ar, ⁴⁰K

Potassium-40

Potassium-40 is a radioactive isotope of potassium which has a very long half-life of 1.248 years, or about 39.38 seconds.Potassium-40 is a rare example of an isotope which undergoes all three types of beta decay. About 89.28% of the time, it decays to calcium-40 with emission of a beta particle...

, and ⁴⁰Ca. The nuclei of these nuclides all contain 40 nucleons, however they contain varying numbers of protons and neutrons.

History of the term

The word "isobar" was coined in 1864. It is derived from the Greek

Greek language

Greek is an independent branch of the Indo-European family of languages. Native to the southern Balkans, it has the longest documented history of any Indo-European language, spanning 34 centuries of written records. Its writing system has been the Greek alphabet for the majority of its history;...

 word isos, meaning "equal" and baros, meaning "weight".

See also

• Atom
  Atom
  The atom is a basic unit of matter that consists of a dense central nucleus surrounded by a cloud of negatively charged electrons. The atomic nucleus contains a mix of positively charged protons and electrically neutral neutrons...
  
  
• Beta-decay stable isobars
  Beta-decay stable isobars
  Beta-decay stable isobars are the set of nuclides which cannot undergo beta decay, that is, the transformation of a neutron to a proton or a proton to a neutron within the nucleus...
  
  
• Table of nuclides
  Table of nuclides
  The tables listed below provide information on the basic properties of all nuclides.* Neutron + Element 1 - Element 24 * Element 25 - Element 48 * Element 49 - Element 72...
  
  
• Radionuclide
  Radionuclide
  A radionuclide is an atom with an unstable nucleus, which is a nucleus characterized by excess energy available to be imparted either to a newly created radiation particle within the nucleus or to an atomic electron. The radionuclide, in this process, undergoes radioactive decay, and emits gamma...
  
   (or radioisotope)
• Nuclear medicine
  Nuclear medicine
  In nuclear medicine procedures, elemental radionuclides are combined with other elements to form chemical compounds, or else combined with existing pharmaceutical compounds, to form radiopharmaceuticals. These radiopharmaceuticals, once administered to the patient, can localize to specific organs...
  
   (includes medical isotopes)
• Isotopomer
• List of particles
• Isobars are nuclides having the same mass number (sum of protons and neutrons); compare:
  • Isotope
    Isotope
    Isotopes are variants of atoms of a particular chemical element, which have differing numbers of neutrons. Atoms of a particular element by definition must contain the same number of protons but may have a distinct number of neutrons which differs from atom to atom, without changing the designation...
    
    s are nuclides having the same atomic number (number of protons).
  • Isotone
    Isotone
    Two nuclides are isotones if they have the same neutron number N, but different proton number Z. For example, boron-12 and carbon-13 nuclei both contain 7 neutrons, and so are isotones. Similarly, S-36, Cl-37, Ar-38, K-39, and Ca-40 nuclei are all isotones of 20 because they all contain 20 neutrons...
    
    s are nuclides having the same number of neutrons.
  • Nuclear isomer
    Nuclear isomer
    A nuclear isomer is a metastable state of an atomic nucleus caused by the excitation of one or more of its nucleons . "Metastable" refers to the fact that these excited states have half-lives more than 100 to 1000 times the half-lives of the other possible excited nuclear states...
    
    s are different excited states of the same type of nucleus. A transition from one isomer to another is accompanied by emission or absorption of a gamma ray
    Gamma ray
    Gamma radiation, also known as gamma rays or hyphenated as gamma-rays and denoted as γ, is electromagnetic radiation of high frequency . Gamma rays are usually naturally produced on Earth by decay of high energy states in atomic nuclei...
    
    , or the process of internal conversion
    Internal conversion
    Internal conversion is a radioactive decay process where an excited nucleus interacts with an electron in one of the lower atomic orbitals, causing the electron to be emitted from the atom. Thus, in an internal conversion process, a high-energy electron is emitted from the radioactive atom, but...
    
     (not to be confused with chemical isomers
    Isomer
    In chemistry, isomers are compounds with the same molecular formula but different structural formulas. Isomers do not necessarily share similar properties, unless they also have the same functional groups. There are many different classes of isomers, like stereoisomers, enantiomers, geometrical...
    
    ).
• Bainbridge mass spectrometer