"C" for carbon, standard notation (now known as "AZE notation" because A is the mass number, Z the atomic number, and E for element) is to indicate the mass number (number of nucleons) with a superscript at the upper left of the chemical symbol and to indicate the atomic number with a subscript at the lower left (e.g. However, isotope is the older term and so is better known than nuclide and is still sometimes used in contexts in which nuclide might be more appropriate, such as nuclear technology and nuclear medicine.Īn isotope and/or nuclide is specified by the name of the particular element (this indicates the atomic number) followed by a hyphen and the mass number (e.g. For example, the nuclides 12Īre isotopes (nuclides with the same atomic number but different mass numbers ), but 40Īre isobars (nuclides with the same mass number ). The term isotopes (originally also isotopic elements, now sometimes isotopic nuclides ) is intended to imply comparison (like synonyms or isomers). Even for the lightest elements, whose ratio of neutron number to atomic number varies the most between isotopes, it usually has only a small effect although it matters in some circumstances (for hydrogen, the lightest element, the isotope effect is large enough to affect biology strongly). The neutron number has large effects on nuclear properties, but its effect on chemical properties is negligible for most elements. The nuclide concept (referring to individual nuclear species) emphasizes nuclear properties over chemical properties, whereas the isotope concept (grouping all atoms of each element) emphasizes chemical over nuclear. The atomic number of carbon is 6, which means that every carbon atom has 6 protons so that the neutron numbers of these isotopes are 6, 7, and 8 respectively.Ī nuclide is a species of an atom with a specific number of protons and neutrons in the nucleus, for example carbon-13 with 6 protons and 7 neutrons. The number of nucleons (both protons and neutrons) in the nucleus is the atom's mass number, and each isotope of a given element has a different mass number.įor example, carbon-12, carbon-13, and carbon-14 are three isotopes of the element carbon with mass numbers 12, 13, and 14, respectively. Each atomic number identifies a specific element, but not the isotope an atom of a given element may have a wide range in its number of neutrons. The number of protons within the atom's nucleus is called its atomic number and is equal to the number of electrons in the neutral (non-ionized) atom. It was coined by Scottish doctor and writer Margaret Todd in 1913 in a suggestion to the British chemist Frederick Soddy. The term isotope is formed from the Greek roots isos ( ἴσος "equal") and topos ( τόπος "place"), meaning "the same place" thus, the meaning behind the name is that different isotopes of a single element occupy the same position on the periodic table. While all isotopes of a given element have almost the same chemical properties, they have different atomic masses and physical properties. They have the same atomic number (number of protons in their nuclei) and position in the periodic table (and hence belong to the same chemical element), but differ in nucleon numbers ( mass numbers) due to different numbers of neutrons in their nuclei. Isotopes are distinct nuclear species (or nuclides, as technical term) of the same element. From left to right, the isotopes are protium ( 1H) with zero neutrons, deuterium ( 2H) with one neutron, and tritium ( 3H) with two neutrons. The fact that each isotope has one proton makes them all variants of hydrogen: the identity of the isotope is given by the number of protons and neutrons. The three naturally occurring isotopes of hydrogen.
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