Question

wait: why do we round average atomic mass? which number is unique for each atom? protons, neutrons or electrons?

Explanation:

1. For rounding average atomic mass: Average atomic mass is a weighted average of an element's isotopes. Rounding it to a whole number approximates the mass number (sum of protons and neutrons) of the most abundant, stable isotope, which is useful for quick stoichiometric calculations and general chemistry contexts where precise decimal values are not necessary.
2. For the unique number and subatomic particles: The atomic number, equal to the number of protons in a neutral atom, is unique to each element. Protons define an element's identity—changing the number of protons changes the element. Neutrons can vary (creating isotopes) and electrons can be gained/lost (creating ions) without changing the element itself.

Answer:

1. We round average atomic mass to approximate the mass number of the most common stable isotope, simplifying basic chemistry calculations.
2. The atomic number (number of protons) is unique for each atom/element. This is because protons determine an element's identity, while neutrons (isotopes) and electrons (ions) can vary without changing the element.