3.1Relative Atomic Mass and Relative Molecular Mass
Atoms and molecules are extremely small and impossible to count or weigh individually. To overcome this difficulty, chemists compare the masses of atoms and molecules to an agreed standard element using the concepts of relative atomic mass (RAM), relative molecular mass (RMM), and relative formula mass (RFM).
Learning Objectives
- Conceptualise relative atomic mass (RAM) and relative molecular mass (RMM) based on the carbon-12 scale.
- Explain why carbon-12 was chosen as the international standard atom to determine relative atomic mass.
- Calculate the relative molecular mass (RMM) of molecular substances by summing up the relative atomic masses of all constituent atoms.
- Calculate the relative formula mass (RFM) of ionic substances using chemical formulae.
Balance Comparison of Masses on the Carbon-12 Scale
Show how masses of atoms and molecules balance with units representing \(\frac{1}{12}\) of a carbon-12 atom.
Brief Explanation
Understand the key ideas behind relative masses.
1. Historical Standard: Hydrogen
Hydrogen was initially chosen because it is the lightest atom, assigned a mass of 1 unit. One carbon atom is as heavy as 12 hydrogen atoms, giving carbon a relative atomic mass of 12. However, using hydrogen encountered various experimental problems.
2. The Carbon-12 Standard
In 1961, chemists worldwide agreed to adopt the carbon-12 atom as the international standard. One atom of carbon-12 is assigned a definite mass of exactly 12 units.
| Reason | Explanation |
|---|---|
| Solid at room temperature | Carbon-12 can be handled easily. |
| Combines easily | It combines easily with other elements and is found in most substances. |
| Major isotope | Carbon-12 is the major isotope of carbon with an abundance of 99%, making its relative atomic mass exactly 12.0. |
Relative Atomic Mass (RAM)
The average mass of an atom of the element compared to \(\frac{1}{12}\) of the mass of one carbon-12 atom.
\[\text{Relative atomic mass of an element} = \dfrac{\text{Average mass of one atom of the element}}{\frac{1}{12} \times \text{Mass of one carbon-12 atom}}\]
Example: The RAM of helium is 4. This means the average mass of one helium atom is 4 times the mass of \(\frac{1}{12}\) of a carbon-12 atom.
Relative Molecular Mass (RMM)
The average mass of a molecule compared to \(\frac{1}{12}\) of the mass of one carbon-12 atom.
\[\text{Relative molecular mass of a substance} = \dfrac{\text{Average mass of one molecule}}{\frac{1}{12} \times \text{Mass of one carbon-12 atom}}\]
Example: A water molecule (H2O) has an RMM of 18, meaning its mass is 18 times the mass of \(\frac{1}{12}\) of a carbon-12 atom.
| Substance | Calculation | Relative mass |
|---|---|---|
| Oxygen, O2 | \(2 \times 16\) | 32 |
| Water, H2O | \(2(1) + 16\) | 18 |
| Glucose, C6H12O6 | \(6(12) + 12(1) + 6(16) = 72 + 12 + 96\) | 180 |
Worked Example: Zinc chloride, ZnCl2
\[\text{RFM} = \text{RAM of Zn} + 2(\text{RAM of Cl})\]
\[= 65 + 2(35.5) = 65 + 71 = 136\]
Worked Example: Aluminium sulphate, Al2(SO4)3
\[\text{RFM} = 2(\text{RAM of Al}) + 3[\text{RAM of S} + 4(\text{RAM of O})]\]
\[= 2(27) + 3[32 + 4(16)] = 54 + 3[96] = 54 + 288 = 342\]
Try These Questions First
Answer mentally, then check the explanation.
Why does relative atomic mass (RAM) have no unit?
One magnesium atom is twice as heavy as one carbon-12 atom. What is the RAM of magnesium?
Calculate the relative formula mass of zinc chloride, ZnCl2, given that the RAM of Zn = 65 and Cl = 35.5.
Common Mistakes
Avoid these errors when working with relative masses.
Common Misconceptions
Compare each misconception with the correct fact.
Carbon-12 was chosen because it is the lightest element.
Hydrogen is the lightest element, but carbon-12 was chosen because it is a solid at room temperature, easily handled, highly abundant (99%), and forms compounds with many other elements.
Losing or gaining electrons changes an ion's mass significantly compared to its neutral atom.
Electrons have negligible mass, so the mass of an ion does not differ significantly from the mass of its parent atom; hence atomic masses are directly summed for RFM.
Summary
Key facts to remember.
- Relative atomic mass (RAM), relative molecular mass (RMM), and relative formula mass (RFM) compare the masses of atoms, molecules, and ionic formula units to \(\frac{1}{12}\) of the mass of a carbon-12 atom.
- Relative masses have no units.
- Carbon-12 is the universal standard, assigned an exact mass of 12 units.
- RMM is used for covalent molecular compounds, while RFM is used for ionic compounds.
- Relative masses are determined by adding together the RAM values of all the atoms in a chemical formula.
Quick Activity
Complete all parts before checking your score.
A. Objective Quiz
What is the standard used internationally since 1961 to determine relative atomic mass?
What is the relative molecular mass (RMM) of carbon dioxide, CO2? [Relative atomic mass: C = 12, O = 16]
B. Fill in the Blanks
Because relative atomic mass is a comparative value comparing an atom to a standard, it has no ______.
The relative mass of an ionic substance calculated by summing the masses of its constituent ions is called relative ______ mass.
C. Match the Term or Substance with Its Description
Drag each term on desktop. On mobile, tap a term and then tap the matching box.
Term / Substance
Carbon-12 Water (H2O) RFM HeliumDescription / Value
Keywords
Tap or hover over a term to read its meaning.
