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The difference between the mass number and the atomic number (A-Z) gives the number of neutrons in a given nucleus.

For example: Carbon-14 is created from Nitrogen-14 with seven protons and seven neutrons via a Cosmic Ray interaction which changes 1 proton into 1 neutron. Thus the atomic number decreases by 1 (Z:7->6) and the mass number remains the same (A = 14), however the number of neutrons increases by 1.
:Before: Nitrogen-14 (7p, 7n)
:After: Carbon-14 (6p, 8n).