Relative masses of atoms and molecules
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Define the unified atomic mass unit (u).
The unified atomic mass unit (u) is defined as one-twelfth of the mass of a carbon-12 atom. It is used as a standard unit for measuring the mass of atoms and molecules.
Define relative atomic mass (Ar).
Relative atomic mass (Ar) is the weighted average mass of the naturally occurring isotopes of an element, relative to one-twelfth of the mass of a carbon-12 atom. It is a dimensionless quantity.
Define relative isotopic mass.
Relative isotopic mass is the mass of an isotope relative to one-twelfth of the mass of a carbon-12 atom. It is a dimensionless quantity.
Define relative molecular mass (Mr).
Relative molecular mass (Mr) is the sum of the relative atomic masses of all the atoms in a molecule, relative to one-twelfth of the mass of a carbon-12 atom. It is a dimensionless quantity.
Define relative formula mass.
Relative formula mass is the sum of the relative atomic masses of all the ions in a formula unit of an ionic compound, relative to one-twelfth of the mass of a carbon-12 atom. It is a dimensionless quantity.
Explain the relationship between relative atomic mass (Ar) and the unified atomic mass unit (u).
Relative atomic mass (Ar) is a ratio. It represents how many times heavier, on average, an atom of an element is compared to 1/12th the mass of a carbon-12 atom (1 u). Therefore, it is dimensionless.
Describe how relative atomic mass (Ar) is calculated from isotopic abundance data.
Ar = Σ (isotopic mass × % abundance) / 100. Multiply the mass of each isotope by its percentage abundance, sum the results, and divide by 100 to account for the percentage. This calculates the weighted average.
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