atomic mass
Summary
Atomic mass is the mass of one atom of a nuclide, measured in atomic mass units (u), where 1 u is 1/12 the mass of a carbon-12 atom (about 1.66 × 10⁻²⁷ kg). It is close to the mass number but slightly less than the sum of its parts, and that mass defect gives the binding energy through E = Δm c².
Detail
Meaning: Atomic mass is the mass of one atom of a given nuclide, measured in atomic mass units. One u is 1/12 the mass of a carbon-12 atom, about 1.66 × 10⁻²⁷ kg. A proton and a neutron each weigh about 1 u and electrons add very little, so atomic mass is close to the mass number A. It is not exactly A, because a nucleus has less mass than its separated nucleons; the difference is the mass defect. (In chemistry, the periodic-table value is a weighted average over isotopes, in g/mol.)
Equation: E = Δm c², where E is the binding energy (J), Δm is the mass defect (kg), and c = 3.00 × 10⁸ m/s. Also, 1 u corresponds to about 931.5 MeV.
On the MCAT: Passages list masses in u for a nucleus and for its decay, fission or fusion products. Find the mass lost, then multiply by 931.5 MeV/u to get the energy released.
Common confusion: Mass number A is a count of nucleons and has no unit, while atomic mass is a measured mass in u. A nucleus is lighter than its separate parts because energy was released when it formed, and that same energy must be supplied to pull it apart.
Sources
- Physics (Cutnell and Johnson)
- University Physics (Young and Freedman)
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.