The video tutorial explains the concept of atomic mass, focusing on subatomic particles like protons and neutrons. It introduces the atomic mass unit (AMU) as a measure for these particles. The tutorial details how to calculate the mass number by adding protons and neutrons, using carbon as an example. It also discusses isotopes, which are atoms of the same element with different neutron counts, affecting the atomic mass. The atomic mass is a weighted average of all isotopes, often resulting in a decimal value.

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