3.1 - Relative Atomic Mass and Relative Molecular Mass

Relative Atomic Mass, RAM

• The mass of an atom is extremely small, so chemists compare it with a standard rather than use its actual mass.
• Hydrogen was initially used as the standard because it is the lightest atom. In 1961, chemists agreed to use the carbon-12 atom instead.
• One carbon-12 atom is assigned a mass of exactly 12 units. Therefore, $\frac{1}{12}$ of its mass is one unit.
• The relative atomic mass, RAM, of an element is the average mass of one atom of the element compared with $\frac{1}{12}$ of the mass of one carbon-12 atom:

• Example: RAM of helium is 4, meaning that the average mass of one helium atom is four times $\frac{1}{12}$ of the mass of one carbon-12 atom.
• RAM is a comparative value and therefore has no unit.
• Carbon-12 is a suitable standard because it:
  • is a solid at room temperature and is easy to handle;
  • combines readily with other elements and occurs in many substances;
  • is the major carbon isotope, with about 99% abundance;
  • has a relative atomic mass of exactly 12.0.

Relative Molecular Mass, RMM

• The relative molecular mass, RMM, of a substance is the average mass of one molecule compared with $\frac{1}{12}$ of the mass of one carbon-12 atom:

• RMM is calculated by adding the RAM values of all atoms shown in a molecule’s formula:

• Examples:
  • Oxygen, ${\mathrm{O}}_2$: $2(16)=32$.
  • Water, ${\mathrm{H}}_2\mathrm{O}$: $2(1)+16=18$.
  • Glucose, ${\mathrm{C}}_6{\mathrm{H}}_{12}{\mathrm{O}}_6$: $6(12)+12(1)+6(16)=180$.
• Like RAM, RMM has no unit.

Relative Formula Mass, RFM

• Ionic substances do not consist of discrete molecules. Their relative mass is called relative formula mass, RFM.
• RFM is the sum of the RAM values of all atoms shown in the formula of an ionic substance. The mass of an ion differs negligibly from the mass of the atom that forms it.
• Examples:
  • Zinc chloride, $\mathrm{ZnC}{\mathrm{l}}_2$: $65+2(35.5)=136$.
  • Aluminium sulphate, $\mathrm{A}{\mathrm{l}}_2(\mathrm{S}{\mathrm{O}}_4{)}_3$:

• RFM is also a unitless comparative value.