Atomic Mass Units (Mass Number)

Estimate the atomic mass of a single atom by summing its nucleons, since electrons contribute negligibly to mass.

$$A=Z+N$$

Variables

Symbol	Meaning	Unit
A	Atomic mass (mass number)	u
Z	Number of protons	count
N	Number of neutrons	count

Worked example

Carbon-12 has 6 protons and 6 neutrons, so A = 6 + 6 = 12 u. Hydrogen has 1 proton and 0 neutrons, so A = 1 u.

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Part of The Blueprint of Water