Isotope definitions

ī'sə-tōp'
The definition of an isotope is an element with similar chemical make-up and the same atomic number, but different atomic weights to another or others.

An example of an isotope is Carbon 12 to Carbon 13.

noun
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One of two or more atoms having the same atomic number but different mass numbers.
noun
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Any of two or more forms of an element having the same or very closely related chemical properties and the same atomic number but different atomic weights (or mass numbers)

U-235, U-238, and U-239 are three isotopes of uranium.

noun
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(physics) Any of two or more forms of an element where the atoms have the same number of protons, but a different number of neutrons within their nuclei. As a consequence, atoms for the same isotope will have the same atomic number but a different mass number (atomic weight).
noun
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(topology) To define or demonstrate an isotopy of (one map with another).
verb
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One of two or more atoms that have the same atomic number (the same number of protons) but a different number of neutrons. Carbon 12, the most common form of carbon, has six protons and six neutrons, whereas carbon 14 has six protons and eight neutrons. Isotopes of a given element typically behave alike chemically. With the exception of hydrogen, elements found on Earth generally have the same number of protons and neutrons; heavier and lighter isotopes (with more or fewer neutrons) are often unstable and undergo radioactive decay .
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One member of a family of chemical elements that has the same chemical properties (the same atomic number) but differs in mass. Isotopes have the same number of protons and electrons, but a different number of neutrons. The mass is determined by the total number of nucleons (neutrons and protons). See allotrope.
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Origin of isotope

Coined in 1914 by British chemist Frederick Soddy from Ancient Greek ἴσος (ísos, “equal; same”) and τόπος (tópos, “place”), because the different isotopes of a chemical element always occupy the same position in the periodic table of elements. Compare the synonymous Icelandic word samsæta.