Biology / Unit 1 Bio 0115 short lesson, about 20 seconds
Atomic Number and Mass Number
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The idea
An atom's atomic number counts its protons, and its mass number counts its protons and neutrons together.
In practice
Carbon's atomic number is six, meaning every carbon atom has six protons, whatever its number of neutrons.
Where people slip
The atomic number identifies the element; the mass number can vary between atoms of the same element.
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