Determine the number of neutrons for the given isotopes:
helium-3
carbon-14
nitrogen-15
strontium-90
tellurium-123

Helium = 1
Carbon = 8
Nitrogen = 8
Strontium = 52
Tellurium = 71

If you look on a periodic table, on each element there is a number on the top left. This represents the number of protons in an atom. Protons have a mass of 1 (in relative to Carbon-13)

If we take nitrogen-15 for example; The number 15 tells you that the isotope has a mass of 15. Now if you look on the periodic table, Nitrogen has a proton number of 7. Only protons and neutrons have a mass, electrons are considered to be negligable. Therefore the number of neutrons Nitrogen-15 contains is 15 - 7 = 8

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