Chemistry happens one particle at a time — this atom meets that one — and particles are far too small to count. So chemists count the only way anyone can: they WEIGH. The bridge between the balance and the headcount is the mole, and it is nothing more exotic than a word meaning a specific number of things, the way “dozen” means twelve.

The number is Avogadro's: $N_A = 6.02 \times 10^{23}$ things per mole. Why that number and not a rounder one? Because it was chosen to make the bookkeeping free — one mole of carbon-12 atoms weighs exactly 12 grams. That single choice ties the periodic table's tidy little numbers directly to your balance, and the whole chapter falls out of it.