Net charge sign is scanned across ion and electron counts. Exact arithmetic here means exact results for the stated model inputs; measured inputs still carry uncertainty and significant-figure limits.
highlighted = computed this step
Net charge is a count difference
The checked source has one more ion than electron, so the finite charge ledger is positive one coulomb.
5⋅1+4⋅(−1)=1C
Adding or removing one charge flips the sign
The middle row has excess ion charge. The last row has excess electron charge. The neutral row has equal counts.