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Provides a simplified description of the atomic nucleus.

What is the question?H-3 (or hydrogen-3, otherwise known as tritium) is an isotope of hydrogen with __________.
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OK	Value	Feedback
	3 neutrons	The 3 in H-3 represents the mass number A = N+Z, i.e. the total number of nucleons... and in this case, A = N+Z = 2+1 = 3.
	2 neutrons	Correct! Since A = N+Z, then N = A-Z = 3-1 = 2.
	2 protons	Hydrogen nuclei always have 1 proton, even if they're isotopes with 0, 1, 2, or more neutrons. (Having 2 protons would make it a nucleus of helium, a different element altogether.)
	3 protons	Hydrogen nuclei always have 1 proton, even if they're isotopes with 0, 1, 2, or more neutrons. (Having 3 protons would make it a nucleus of lithium, a different element altogether.)

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