Unified atomic mass unit, u

· Unified atomic mass unit, symbol u, is defined as one twelfth of the mass of one carbon-12 atom.
· Carbon-12 is used as the standard reference for comparing atomic and molecular masses.
· 1 carbon-12 atom has a relative mass of exactly 12 u, so 1 u = 1/12 × mass of a carbon-12 atom.
· Relative masses are ratios, so values such as Ar, relative isotopic mass, Mr and relative formula mass have no units.
· Exam phrasing: always refer to one atom of carbon-12, not one mole of carbon-12.

Carbon-12 is the reference atom used to define the unified atomic mass unit. Since carbon-12 is assigned a relative mass of exactly 12, one unified atomic mass unit is one twelfth of this mass. Source

Relative isotopic mass

· Relative isotopic mass = the mass of one atom of an isotope compared with 1/12 of the mass of one carbon-12 atom.
· It applies to a specific isotope, e.g. carbon-12, carbon-13, chlorine-35 or chlorine-37.
· It is a relative value, so it has no units.
· Do not confuse relative isotopic mass with relative atomic mass: isotopic mass refers to one isotope, while atomic mass usually considers the naturally occurring isotopic mixture.

Relative atomic mass, Ar

· Relative atomic mass, Ar = the weighted mean mass of one atom of an element compared with 1/12 of the mass of one carbon-12 atom.
· Weighted mean means isotopes with greater natural abundance contribute more to the final value.
· Ar values are often decimals because most elements exist as a mixture of isotopes.
· In exam definitions, include all three key ideas: weighted mean, atoms of an element, and relative to 1/12 of carbon-12.
· Ar has no units because it is a comparison ratio.

Relative molecular mass, Mr

· Relative molecular mass, Mr = the mass of one molecule compared with 1/12 of the mass of one carbon-12 atom.
· Used for substances made of simple molecules, e.g. H₂O, CO₂, NH₃, CH₄.
· Calculate Mr by adding the Ar values of all atoms in the molecule.
· Example method: Mr(CO₂) = Ar(C) + 2 × Ar(O).
· Mr has no units because it is a relative mass.

These diagrams show that molecular mass is found by summing the atomic masses of every atom in a molecule. They are useful for linking a displayed molecular structure to an Mr calculation. Source

Relative formula mass

· Relative formula mass = the mass of one formula unit compared with 1/12 of the mass of one carbon-12 atom.
· Usually used for ionic compounds or giant structures where there are no separate molecules, e.g. NaCl, MgO, CaCO₃.
· Calculate it by adding the Ar values of all atoms shown in the formula.
· Example method: relative formula mass of MgCl₂ = Ar(Mg) + 2 × Ar(Cl).
· Avoid calling ionic compounds “molecules”; use formula unit instead.

Exam definition traps

· Do not write “mass of carbon-12” only; write one twelfth of the mass of one carbon-12 atom.
· Do not include units for Ar, Mr, relative isotopic mass or relative formula mass.
· Relative isotopic mass refers to one isotope; relative atomic mass refers to the weighted mean of an element’s isotopes.
· Mr is for molecules; relative formula mass is for formula units, especially ionic compounds.
· In definitions, the comparison must always be to 1/12 of a carbon-12 atom.