The mass of an atom of phosphorus relative to the mass of a carbon-12 atom can be determined using the concept of atomic mass units (amu) or unified atomic mass unit (u). The atomic mass of carbon-12 is defined as exactly 12 atomic mass units.
The atomic mass of phosphorus is approximately 30.974 u. Therefore, the mass of a phosphorus atom relative to a carbon-12 atom is approximately 30.974 u divided by 12 u, which equals 2.581.
In other words, a phosphorus atom is approximately 2.581 times heavier than a carbon-12 atom.