What is the Difference Between Atomic Weight and Atomic Mass?

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The main difference between atomic weight and atomic mass lies in the fact that atomic weight is the average mass of all naturally occurring isotopes of an element, while atomic mass represents the mass of a single isotope or single atom. Here are some key differences between the two:

  • Atomic Mass: This is the mass of a single atom or an individual isotope. It is independent of the atomic masses of isotopes and can be calculated using the mass of neutrons and protons of an atom. The calculated value is a whole number.
  • Atomic Weight: This is the average weight of an element with respect to all its isotopes and their relative abundances. It is dependent on the atomic mass of isotopes and their abundance. To calculate atomic weight, it is necessary to consider the masses of isotopes and their percentage in nature. The calculated value may or may not be a whole number.

Both atomic mass and atomic weight are measured in units of atomic mass (usually abbreviated to AMU), also known as Daltons. The atomic mass of elements differs as it depends on the number of protons and neutrons in an atom.

Comparative Table: Atomic Weight vs Atomic Mass

Here is a table highlighting the differences between atomic weight and atomic mass:

Atomic Mass Atomic Weight
Independent of the atomic masses of isotopes Dependent on the atomic mass of isotopes and their abundance
Calculated using the mass of neutrons and protons of an atom Calculated as a weighted average of the mass of all the atoms of an element, based on the abundance of isotopes
The calculated value is a whole number The calculated value may or may not be a whole number
Represents the mass of an atom in Da (Dalton) Represents the average weight of an element with respect to all its isotopes

Atomic mass is the mass of a single atom of an element, while atomic weight is a weighted average of the masses of all the atoms of an element, based on the abundance of isotopes. To calculate atomic weight, you need to consider isotope relative abundance and isotope mass.