How is atomic number different from mass number?

Hello Gary My Man!

Well, as you can clearly see 

The atomic number of an element is basically the number of protons it has. So yes, for every element this is different. Now, the mass number of an element as known, is the number of protons+the number of neutrons.  So theoretically as we can see, this number should be a whole number, but since there are different isotopes (atoms of the same element with different numbers of neutrons) of each element, most periodic tables take account of that, so they often include decimals as seen. 

So in Short, ALL the atoms of a particular element have the SAME EXACT atomic number (number of protons of course). The atoms of different elements have very different numbers of protons. And of course, the MASS number of an atom is the TOTAL number as known, of protons and of course, the neutrons it contains in it.

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The answer is b because a,c,and d are opinons