Atomic or nuclear mass (weight)

Understanding how to determine atomic (nuclear) mass or calculate molecular weight is one of the most basic and most important skills in chemistry. This skill opens a whole new horizon of possibilities, the most obvious of which is determining molecular or molar mass which you are yet to discover in future articles of this manual.

So let’s take a look at what actually is an atomic mass and how to determine it it.

Since a single atom or a single molecule are so miniscule that common accepted mass units like gram and kilogram are a bit inconvenient. So for starters let’s look at some additional units of measure, used in chemistry. I assure you, they are not complicated to grasp.

What is the Unified atomic mass unit

Unified atomic mass is the mass, expressed in relation to 1/12th part of carbon-12. Confusing, isn’t it? Now let’s put it more simply. The most abundant carbon isotope is carbon 12. Carbon atom has 6 protons in its nucleus, and carbon 12 in particular has also exactly 6 neutrons, so carbon 12 has exactly 12 particles in the nucleus. Therefore, Unified atomic mass unit (or Dalton) is a mass that approximately equals to a mass of a single proton or a single neutron. It is considered that the mass of a single proton or a single neutron equals to 1 Da (one Dalton).

Thus the mass of a carbon 12 atom is a combined mass of 6 protons and 6 neutrons and equals to 6 + 6 = 12 Da.

Atomic mass can be always found in any periodic table of elements. Look at a particular table legend and you will know where the mass is.

When you look at a periodic table of elements, you might have a reasonable question: If atomic weight of an atom is pretty much a total number of its nucleons, then why is atomic weight shown in periodic tables is always fractional? For instance, atomic weight of Carbon is 12,011. It just can’t be that calcium atom has 12 nucleons and some 0,011 of a proton or neutron in its nucleus, right?

Right! It is like this because a simple substance of any element consists of different isotopes, with some of them slightly heavier or lighter (having more or lacking certain amounts of neutrons). And these isotopes are present at varying ratios. The mass written in periodic tables takes that fact into consideration and mass specified there is an average calculated with the consideration of isotope content of an element.

So your takeaway here is pretty straightforward. Whenever you are asked to determine atomic weight of a particular element, you are not supposed to actually calculate anything, you simply need to refer to a periodic table of elements and properly read the mass from there. Thankfully, it is pretty unheard of that students would actually be forced to memorise those numbers, since it would be pretty pointless and would not contribute to a student’s understanding of chemistry’s beauty.

And if you are asked to determine atomic mass of a particular isotope then it is very easy, because the actual name of isotope indicates this isotope’s nuclear mass. The weight of calcium 40 atom is actually 40. And the weight of oxygen 16 is… well, you probably guessed it is 16.

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