Mass Number
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The mass number is the total protons plus neutrons in an atom's nucleus; subtracting the atomic number gives the neutron count.
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An atom with atomic number 17 and mass number 35 has 17 protons and 18 neutrons, since 35 minus 17 equals 18.
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Mass number is a count, not a mass in grams — it ignores electrons and slight proton-neutron mass difference.