The terms "relative atomic mass," "relative molecular mass," and "relative formula mass" are related but have specific applications in different contexts. Here's an explanation of each term:
Relative Atomic Mass (Ar): Relative atomic mass refers to the average mass of an atom of an element compared to 1/12th the mass of a carbon-12 atom. It is a dimensionless quantity expressed as a ratio or multiple of atomic mass units (u). The relative atomic mass takes into account the natural abundance of isotopes of an element and their respective masses. It is calculated by considering the masses and abundances of all naturally occurring isotopes of an element and averaging them according to their relative abundance. The relative atomic mass is denoted by the symbol "Ar." For example, the relative atomic mass of carbon is approximately 12.01 u, indicating that on average, a carbon atom is 12.01 times as heavy as 1/12th the mass of a carbon-12 atom.
Relative Molecular Mass (Mr): Relative molecular mass, also known as molecular weight, is the sum of the relative atomic masses of all the atoms in a molecule. It represents the mass of a molecule relative to 1/12th the mass of a carbon-12 atom. It is calculated by adding up the relative atomic masses of all the atoms in a molecule. The relative molecular mass is denoted by the symbol "Mr." For example, the relative molecular mass of water (H2O) is approximately 18.02 u because it consists of two hydrogen atoms (each with a relative atomic mass of approximately 1.01 u) and one oxygen atom (with a relative atomic mass of approximately 16.00 u).
Relative Formula Mass: Relative formula mass is used when referring to ionic compounds or substances that do not exist as discrete molecules. It is the sum of the relative atomic masses of all the atoms in the empirical formula of the compound. The empirical formula represents the simplest ratio of elements in the compound. The term "relative formula mass" is commonly used for inorganic compounds and ionic substances because these compounds do not have distinct molecules with specific molecular masses. Instead, they consist of repeating units called formula units. The relative formula mass is calculated by summing the relative atomic masses of all the atoms in the empirical formula. For example, the relative formula mass of sodium chloride (NaCl) is approximately 58.44 u, obtained by adding the relative atomic masses of sodium (Na) and chlorine (Cl).
In summary, the key differences between these terms lie in the type of entity being considered. Relative atomic mass pertains to individual atoms of an element, relative molecular mass applies to molecules, and relative formula mass is used for compounds or substances without distinct molecules, such as ionic compounds.