# Placing the atomic mass and number of an element… [closed]

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

## closed as unclear what you're asking by bon, J. LS, Curt F., M.A.R., YodaApr 21 '15 at 20:49

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• what do you mean by flipped here – DSinghvi Apr 21 '15 at 18:36
• 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. – M.A.R. Apr 21 '15 at 18:44

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:
$\ce{^{14}_6C -> ^{14}_7N} + e^- + \bar\nu_e$