Atomic weight, also known as atomic mass, is a value that represents the average mass of the atoms of an element. It is expressed in atomic mass units (amu) and is based on the atomic masses of the isotopes of that element and their relative abundance in nature.
The atomic mass of an element is determined by considering the masses of its isotopes and their respective abundance in a naturally occurring sample. Isotopes are atoms of the same element that have different numbers of neutrons, resulting in different atomic masses.
To calculate the atomic weight, scientists take into account the atomic masses of all the isotopes of an element and multiply each isotope's mass by its relative abundance (expressed as a decimal or percentage). The individual results are then summed to obtain the atomic weight.
For example, carbon has two stable isotopes: carbon-12 and carbon-13. Carbon-12 is more abundant in nature, accounting for about 98.9% of carbon atoms, while carbon-13 makes up the remaining 1.1%. To calculate the atomic weight of carbon, the atomic masses of carbon-12 and carbon-13 are multiplied by their respective abundances, and the results are added together:
(12 amu × 0.989) + (13 amu × 0.011) = 12.01 amu
Thus, the atomic weight of carbon is approximately 12.01 atomic mass units.
It's worth noting that atomic weights listed on the periodic table are typically average values since the isotopic composition can vary in different samples of an element.