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isotope

Different species of atomic nuclei which contain the same number of protons (and thus represent the same chemical element), but different numbers of neutrons, are called isotopes.

Thus, helium-4 (two protons, two neutrons) and helium-3 (two protons, one neutron) are isotopes, but helium-3 and tritium (one proton, two neutrons) are not. The word can also be used of a specific species of nucleus and a generic chemical element, as in “helium-3 is an isotope of helium”.

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proton

Particle that carries positive electric charge and is comparatively massive; the atomic nuclei consist of protons and neutrons. Protons are not elementary particles, they are compound particles consisting of quarks that are bound together through the strong nuclear interaction . Collectively, protons, neutrons and a number of similar particles are called baryons.

nucleus

Extremely dense central region of an atom, consisting of protons and neutrons held together by nuclear forces. The number of protons determines what chemical element a nucleus represents. Typical diameter for atomic nuclei are in the region of a quadrillionth of a metre= 10^-15 metres. This makes nuclei about a hundredth of a thousandth as large as atoms.

neutrons (neutron)

Particle that is electrically neutral and comparatively massive; the atomic nuclei consist of neutrons and protons. Neutrons are not elementary particles, they are compound particles consisting of quarks that are bound together through the strong nuclear interaction . Collectively, neutrons, protons and a number of similar particles are called baryons. Neutron stars are mainly made of neutrons.

isotope

Different species of atomic nuclei which contain the same number of protons (and thus represent the same chemical element), but different numbers of neutrons, are called isotopes. Thus, helium-4 (two protons, two neutrons) and helium-3 (two protons, one neutron) are isotopes, but helium-3 and tritium (one proton, two neutrons) are not. The word can also be used of a specific species of nucleus and a generic chemical element, as in "helium-3 is an isotope of helium".

helium

After hydrogen, the second lightest chemical element. Its atomic nucleus consists of two protons and, ordinarily, two neutrons ("helium-4"); such helium nuclei are also called alpha particles. Another variety of helium, helium-3, has only one neutron in its nucleus. In the context of general relativity, both helium-3 and helium-4 are is of interest as two species of light atomic nuclei that formed in the early universe during Big Bang Nucleosynthesis.

chemical element (element, chemical)

A substance that cannot be decomposed into more elementary constituent substances by the methods of chemistry. From a physics point of view, to every chemical element corresponds a specific atom which is defined uniquely by the number of protons in its atomic nucleus (for example: nuclei with a single proton define the element hydrogen, two protons define helium, three Lithium, 26 iron and 92 uranium).

chemical element

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