Relative masses
| English | Chinese | Pinyin |
|---|---|---|
| atomic mass unit | 原子质量单位 | yuán zi zhì liàng dān wèi |
| relative atomic mass | 相对原子质量 | xiāng duì yuán zi zhì liàng |
| relative isotopic mass | 相对同位素质量 | xiāng duì tóng wèi sù zhì liàng |
| relative molecular mass | 相对分子质量 | xiāng duì fèn zǐ zhì liàng |
| relative formula mass | 相对式量 | xiāng duì shì liàng |
| ionic compound | 离子化合物 | lí zi huà hé wù |
Weighing the unweighable
- Atoms are far too light to weigh in grams.
- So we compare every mass to one standard: carbon-12.
- This gives simple numbers for atoms, molecules and compounds.
Relative mass lab
relative atomic mass = weighted mean
Change isotope abundance and see the weighted mean move.
The weighted mean mass of an element's atoms, compared to carbon-12, is the relative ______ mass.
Ar (relative atomic mass) accounts for the abundance of each isotope.
The standard and $A_r$
- The unified atomic mass unit 原子质量单位 (u) is exactly $\tfrac{1}{12}$ of the mass of a carbon-12 atom.
- The relative atomic mass 相对原子质量 $A_r$ is the average mass of an element's atoms (over all isotopes, weighted by abundance), compared with $\tfrac{1}{12}$ of carbon-12.
- The relative isotopic mass 相对同位素质量 is the mass of one isotope, same comparison.
A laboratory balance measures mass for mole calculations
The relative atomic mass (Ar) of an element is:
Ar is the average over all isotopes (weighted by abundance), measured against 1/12 of a carbon-12 atom.
$M_r$ and formula mass
- The relative molecular mass 相对分子质量 $M_r$ is the sum of the $A_r$ of all atoms in a molecule.
- The relative formula mass 相对式量 is the same idea for substances that aren't molecules (e.g. ionic compounds 离子化合物) — add the $A_r$ shown in the formula.
- Example: $M_r(\text{CO}_2) = 12 + (2 \times 16) = 44$.
The relative molecular mass (Mr) is found by:
Mr is the sum of the Ar values of every atom in the molecule.
What is the Mr of CO₂? (Ar: C = 12, O = 16)
Mr = 12 + (2 × 16) = 44.
What is the Mr of water, H₂O? (Ar: H = 1, O = 16)
Mr = (2 × 1) + 16 = 18.
Match each term to its meaning.
All relative masses are compared to ¹/₁₂ of a carbon-12 atom; molecules use Mr, ionic compounds use formula mass.
Ionic compounds use a relative FORMULA mass rather than a molecular mass, because they are not made of molecules.
An ionic compound is a giant lattice, not separate molecules, so we add the Ar values shown in its formula.
Keep your masses straight
- Relative atomic mass (Ar) is for atoms; relative molecular/formula mass (Mr) is the sum of the Ar values in a formula.
- Always read isotope data carefully — abundances must add up to 100%.
You've got it
- masses are compared to $\tfrac{1}{12}$ of a carbon-12 atom (the unit u)
- $A_r$ = weighted average over isotopes; relative isotopic mass = one isotope
- $M_r$ = sum of $A_r$ of all atoms; formula mass = same for ionic compounds
- e.g. $M_r(\text{CO}_2) = 44$