Beta-minus decay changes a neutron into a proton in the nuclear ledger, so the mass number stays fixed while the proton count rises. The count change is exact for the stated beta-minus model; real observations infer it through spectra, daughter products, branching probabilities, and detector counts with uncertainty.
Beta-minus decay changes Z while keeping A fixed. Exact arithmetic here means exact results for the stated model inputs; measured inputs still carry uncertainty and significant-figure limits.
highlighted = computed this step
Beta-minus keeps mass number fixed
The parent and daughter both have A 14, but Z increases from 6 to 7.
14=146+1=7
One neutron becomes one proton in the nucleus ledger
The neutron count moves from 8 to 7 as the proton count rises.