Instead, we use the term relative formula mass. Ionic compounds such as sodium chloride and magnesium oxide do no exist as molecules.Īs such, it is confusing to use the term relative molecular mass for ionic compounds. The relative formula mass of an ionic compound is the average mass of one unit of that ionic compound compared to 1/12 of the mass of one carbon-12 atom. Note that for mole calculations, we tend to just use the relative atomic masses (average values) of the elements (as seen in the Periodic Table) and calculate the relative molecular masses (average values). Oxygen is known to exist in three isotopes: oxygen-16, oxygen-17 and oxygen-18.Īs such, we can have carbon dioxide, CO 2 molecules with different molecular masses. To calculate the relative molecular mass of a molecule, you need to add together all the relative atomic masses of all the atoms in its chemical formula.ĭue to isotopic effects, actual molecular masses could be different when the atoms of each element present are isotopes.Ĭarbon is known to exist in two isotopes: carbon-12 and carbon 13. Relative molecular mass is also a ratio and therefore has no unit. The symbol for relative molecular mass is Mr. The mass of a molecule (element or compound) is measured in terms of its relative molecular mass. Many elements and compounds exist as covalent molecules. The relative molecular mass of a molecule is the average mass of one molecule of that element or compound compared to 1/12 of the mass of one carbon-12 atom. Hence, the relative atomic mass of chlorine = (75/100 x 35) +(25/100 x 37) = 35.5 (as shown in the Periodic Table) Let’s take a look at a classic example on how relative atomic mass is being calculated.Ĭhlorine exists as chlorine-35 (% abundance of 75%) and chlorine-37 (% abundance of 25%). The Periodic Table used by GCE A-Level H2 Chemistry students in Singapore The following Periodic Table is used by GCE O-Level Chemistry students. So, relative atomic mass is basically an average value after considering all the different isotopes of the element.įor GCE O-Level Pure Chemistry students (syllabus code 6092) in Singapore, the Periodic Table is not as precise as the one used by the GCE A-Level H2 Chemistry students (syllabus code 9729). The relative atomic masses that you see in the Periodic Table are calculated based on the relative percentage abundance of the isotopes. These elements exist as a mixture of isotopes (which have different mass numbers). If you refer to the Periodic Table, you will notice that the relative atomic masses of some elements are not whole numbers.
Relative atomic mass is a ratio and therefore has no unit.
The symbol for relative atomic mass is Ar. To be more precise, the masses of all elements (in terms of atom) listed in The Periodic Table are always compared to 1/12 of the mass of one atom of carbon-12. The relative atomic mass of an atom is the average mass of one atom of that element compared to 1/12 of the mass of one carbon-12 atom.īasically, it is not practical for scientists to use actual masses of atoms in scientific calculations since atoms have very small masses.Īs such, scientists compare masses of different atoms with reference to the carbon-12 atom (which is an isotope of carbon). Let’s take a look at them one at a time…. In order to differentiate them (as well as see how they are connected to each other), we have to know their definitions first. Many students are confused when it comes to the difference between the following terms commonly used in Mole Concept & Chemical Calculations, namely: