Why do we measure the mass of ten pennies instead of the mass of one penny ?

It really depends on the circumstance, but most likely because every penny doesn’t have the exact same mass, as some have scrapes/scratches, or are chipped. Which result in a different mass, so if you want to know the exact mass of a specific 10 penny’s, you can’t weigh one and multiply it by 10. If you do it’ll be really close, but not exact.
Hope this helps!

Both models are effective because they show the valence electrons, and these valence electrons indicate the reactivity of an element.

The total number of nucleons