Scientists determine the relative atomic mass of elements by comparing the mass of an element's atom to a standard reference isotope. Carbon-12 (C-12) is often used as the reference isotope because it is abundant and has a defined atomic mass of exactly 12 atomic mass units (amu).
Here's a general overview of how scientists calculate the relative atomic mass:
Collect Isotopic Data: Scientists analyze samples of the element and determine the isotopic composition, which refers to the relative abundance of different isotopes of that element. Isotopes are atoms of the same element that have different numbers of neutrons in their nuclei, thus varying in atomic mass.
Calculate Weighted Average: Using the isotopic data, scientists calculate the weighted average of the atomic masses of the different isotopes present in the element. The weighting is based on the relative abundance of each isotope in nature.
Compare to Carbon-12: Next, scientists compare the calculated weighted average to the atomic mass of carbon-12 (12 amu). The ratio of the element's atomic mass to the atomic mass of carbon-12 gives the relative atomic mass of the element.
For example, let's consider chlorine (Cl) which has two stable isotopes, chlorine-35 (75.77% abundance) and chlorine-37 (24.23% abundance). The calculation would be as follows:
(35 amu × 0.7577) + (37 amu × 0.2423) = 35.5 amu
The relative atomic mass of chlorine is 35.5 amu, meaning it is 35.5 times as heavy as 1/12th the mass of a carbon-12 atom.
Scientists use highly accurate instruments like mass spectrometers to measure the masses of atoms and isotopes, which enables precise determinations of relative atomic masses.
It's important to note that the relative atomic mass is a dimensionless quantity and is different from the atomic mass in grams per mole (molar mass). The molar mass is obtained by scaling the relative atomic mass using Avogadro's number and is expressed in grams per mole.