Isotopes are different atoms of the same element whose atoms have the same number of protons but different numbers of neutrons. For example, protium, deuterium and tritium all have 1 proton in their nuclei, but they have 0 neutrons, 1 neutron and 2 neutrons in their nuclei, so they are isotopes of each other.
Of these, protium has a relative atomic mass of 1.007947, deuterium has a relative atomic mass of 2.274246, and tritium has a relative atomic mass of 3.023548, with deuterium being almost twice as heavy as protium, and tritium being almost twice as heavy as protium.
Isotopes are one of two or more atoms of the same chemical element with the same atomic number, occupying the same position on the periodic table and having nearly identical chemical properties.
But the atomic masses or mass numbers, and thus their mass spectral properties, radioactive transitions, and physical properties (e.g., diffusion ability in the gaseous state) differ. Isotopes are indicated by the mass number in the upper left-hand corner of the element's symbol (e.g., carbon 14, which is generally denoted by 14C).
Extended information
Many elements in nature have isotopes. Isotopes are either naturally occurring or artificially created, and some are radioactive or not.
The isotopes of the same element have different mass numbers, but their chemical properties are basically the same (e.g., chemical reactions and the formation of ions), and there are differences in physical properties [mainly in mass (e.g., melting point and boiling point). In nature, various isotopes have a certain percentage of atomic numbers.
Isotopes are atoms (nuclides) that have the same nuclear charge but different atomic masses. Radioactive isotopes were first discovered in the late 19th century, followed by the discovery of naturally occurring stable isotopes and the determination of their abundance. Several stable isotopes exist for most natural elements. The various isotopes of the same element differ in mass but are nearly identical chemically.
Since the discovery of radioactivity at the end of the 19th century, more than 30 radioactive elements have been discovered by the beginning of the 20th century, and it has been demonstrated that some of them are chemically identical, although their radioactivity is significantly different.
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