Question #d725a

You know that the atom is sodium.

If you look in the Periodic Table, you see that #"Na"# has Atomic Number 11.

That tells you that the atom has 11 protons and 11 electrons.

#"mass number = number of protons + number of neutrons"#

#"23 = 11 + number of neutrons"#

∴ #"number of neutrons = 23 – 11 = 12"#

Your atom contains 11 electrons, 11 protons, and 12 neutrons.

Your diagram might look something like this:

The same mass number often corresponds to several isotopes of different elements, having different numbers of protons, neutrons and electrons.
For example, if your mass number is 50, you may have
Titanium 50: 22 protons, 28 neutrons, 22 electrons
Vanadium 50: 23 protons, 27 neutrons, 23 electrons
Chromium 50: 24 protons, 26 neutrons, 24 electrons

These are three stable atoms representing respectively the 5.4 %, 0.25% and the 4.3 % of natural distribution of the three elements.

In the example you gave with sodium you didn't know "just the mass number A", but also the atomic number Z, i.e. the element (sodium) with its characteristic number of protons.

You can find other cases of diads or triads of stable atoms belonging to different elements with the same A, from this graph by searching two or three black dots aligned in a descending diagonal.