Chemistry / Unit 2 Chem 0219 short lesson, about 20 seconds
Mass Number
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The idea
The mass number is the total protons plus neutrons in an atom's nucleus; subtracting the atomic number gives the neutron count.
In practice
An atom with atomic number 17 and mass number 35 has 17 protons and 18 neutrons, since 35 minus 17 equals 18.
Where people slip
Mass number is a count, not a mass in grams — it ignores electrons and slight proton-neutron mass difference.
Your place on the trail
Every lesson rests on earlier ones and opens later ones. Tap any to jump.
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