Why mass number denominates "A" and atomic number - "Z" ?

 

I couldn't find it anywhere. Maybe "A" derivated from some Latin word starts which "A"?

I think it is just A for Atomic mass number.

 

As for Z:

 

The conventional symbol Z comes from the German word Zahl meaning number/numeral/figure, which, prior to the modern synthesis of ideas from chemistry and physics, merely denoted an element's numerical place in the periodic table, whose order is approximately, but not completely, consistent with the order of the elements by atomic weights. Only after 1915, with the suggestion and evidence that this Z number was also the nuclear charge and a physical characteristic of atoms, did the word Atomzahl (and its English equivalent atomic number) come into common use in this context.

https://en.wikipedia.org/wiki/Atomic_number