Why are the atomic and the mass numbers of an element flipped in the textbooks relative to the periodic table?

  • $\begingroup$ what do you mean by flipped here $\endgroup$ – DSinghvi Apr 21 '15 at 18:36
  • $\begingroup$ Welcome to chemistry.SE! This is a bit unclear. Do you mean why is the info about A and Z about every element is included in "books about periodic table"? Also, it would help if you clarify what books you exactly mean. $\endgroup$ – It's Over Apr 21 '15 at 18:44

In the periodic table, the largest number is indeed the atomic number. This is the number of protons. Since this defines the element, it is considered the most important number.

What you probably mean is $^A_Z\mathrm{Element}$. Here, by convention the lower $Z$ (from the German word Zahl is the atomic number, while $A$ is the sum of protons and neutrons. This defines a particular isotope. This notation is often used in the context of NMR spectroscopy ($^{13}\ce{C}$ NMR), or when radioactive decays are described:

\[\ce{^{14}_6C -> ^{14}_7N} + e^- + \bar\nu_e\]

In summary, I think this is just a set of different conventions which show that in differents contexts either the element itself or a particular isotope and its properties are considered more important.


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