Final answer:
The atomic number (Z) equals the number of protons, the mass number (A) equals the number of protons plus neutrons, and you calculate the number of neutrons by subtracting Z from A. For ions, the electron count differs from Z depending on the charge of the ion.
Step-by-step explanation:
To determine the number of protons, neutrons, and electrons in an atom or ion by using the mass number or the A-Z-X symbol, we utilize a few straightforward equations. The atomic number (Z) is equivalent to the number of protons in an atom, while the mass number (A) is the sum of the number of protons and neutrons. Therefore, we can calculate the number of neutrons as A-Z. For an ion with a charge, the number of electrons will be equal to the number of protons (Z) minus the charge if the ion is positive, or plus the charge if the ion is negative.
For example, an oxygen atom with 8 protons (Z=8) and 8 neutrons will have a mass number A of 16 u, thereby giving us the symbol ^16O. Similarly, for a potassium atom with 19 protons and 20 neutrons, we have A=39, resulting in the symbol ^39K. A lithium atom with 3 protons and 4 neutrons will have A=7, and thus the symbol ^7Li. These symbols fully describe the atom, including its atomic number and mass number, providing full information about the isotopes of each element.