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Converting Atomic Percent to Weight Percent and Vice Versa


Plasmaterials, Inc. is a leader in providing high purity materials for all types of thin film applications. We produce and market a full range of products for R&D and full scale production.

A good portion of our customer base here at Plasmaterials, Inc. are scientists, engineers, educators or people working in some form of research and development, be it either in academia or industrial oriented. Such customers are more often concerned with material properties on an atomic level when dealing with elemental materials, alloys and composites. Therefore, they generally think along the lines of atomic interactions from one element to another, how a certain stoichiometric property alters as the composition is changed or an additive is introduced. So, in general, they deal on an Atomic Percentage (At. %) level when considering such properties. Accordingly, we generally receive requests for quotations in atomic percentages or molecular percentages (Mol. %). Unless otherwise stated, liquids and solids are generally expressed in weight percentages and gasses are expressed in volume (Vol. %) or molecular percentages. The exception is within the scientific community where atomic percentages are the norm. This is the area where we at Plasmaterials are often dealing.

Atomic percentages are all well and good when the concern is how individual atoms from one species or another provides a solution to a problem according to how they interact on an atomic level, i.e. when an optical, chemical, electrical, physical, etc. evaluation is being considered. However, for actual manufacturing we cannot measure individual atoms very easily. Therefore, it is necessary for us to deal in weight percentages (Wt. %) when manufacturing an alloy or composite material. It is a lot easier to weigh out the individual constituents in grams, or the like, rather than in atoms.

We have all learned how to figure out how many atoms of a particular element are in a gram back in school. Using a measurement referred as a mole we can use Avogadro’s number and atomic mass units and molar mass units and the like to determine the actual number of atoms there are in a gram of a particular material. So it is possible construct alloys and compounds using individual atom counts, but if you don’t want to deal in numbers in the 10 to the 23rd power range it is a lot easier to deal with atomic weights or molecular weights.

Therefore, it is necessary to convert from At. % to Wt. % or from Wt. % to At. %. This can be done quite easily by using the atomic weight (At. Wt.) or molecular weight (Mol. Wt.) of the constituents. By looking up the atomic or molecular weights of the individual element or molecular constituents to as many significant figures as desired, it is possible to make the simple conversions from one to the other. For example, given two elemental constituents, lets just call them “x” and “y”, we can convert from At.% to Wt. % and Wt. % to At% accordingly.

It is easy to calculate the At. % of y in the same manner by recalculating after substituting all the x’s and y’s or by simply subtracting the Wt. % of x as determined above from 100%.

Conversely to convert from Wt. % to At. %:

As an example, let’s take a standard (Ni)80(Cr)20 Wt. % metallic alloy and see what the At. % would be.

The two decimal point atomic weights for Nickel and Chromium are:

Calculating for At. % Nickel gives:

Calculating for Chromium gives:

So, it is really quite simple to convert from atomic percent to weight percent and vice versa.